Mcm2 hypomorph leads to acute leukemia or hematopoietic stem cell failure, dependent on genetic context.

Minichromosome maintenance proteins (Mcm2-7) form a hexameric complex that unwinds DNA ahead of a replicative fork. The deficiency of Mcm proteins leads to replicative stress and consequent genomic instability. Mice with a germline insertion of a Cre cassette into the 3'UTR of the Mcm2 gene (designated Mcm2Cre ) have decreased Mcm2 expression and invariably develop precursor T-cell lymphoblastic leukemia/lymphoma (pre-T LBL), due to 100-1000 kb deletions involving important tumor suppressor genes. To determine whether mice that were protected from pre-T LBL would develop non-T-cell malignancies, we used two approaches. Mice engrafted with Mcm2Cre/Cre Lin- Sca-1+ Kit+ hematopoietic stem/progenitor cells did not develop hematologic malignancy; however, these mice died of hematopoietic stem cell failure by 6 months of age. Placing the Mcm2Cre allele onto an athymic nu/nu background completely prevented pre-T LBL and extended survival of these mice three-fold (median 296.5 vs. 80.5 days). Ultimately, most Mcm2Cre/Cre ;nu/nu mice developed B-cell precursor acute lymphoblastic leukemia (BCP-ALL). We identified recurrent deletions of 100-1000 kb that involved genes known or suspected to be involved in BCP-ALL, including Pax5, Nf1, Ikzf3, and Bcor. Moreover, whole-exome sequencing identified recurrent mutations of genes known to be involved in BCP-ALL progression, such as Jak1/Jak3, Ptpn11, and Kras. These findings demonstrate that an Mcm2Cre/Cre hypomorph can induce hematopoietic dysfunction via hematopoietic stem cell failure as well as a "deletor" phenotype affecting known or suspected tumor suppressor genes.

Cruise Ship Travel in the Era of Coronavirus Disease 2019 (COVID-19): A Summary of Outbreaks and a Model of Public Health Interventions.

BACKGROUND: Cruise travel contributed to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission when there were relatively few cases in the United States. By 14 March 2020, the Centers for Disease Control and Prevention (CDC) issued a No Sail Order suspending US cruise operations; the last US passenger ship docked on 16 April. METHODS: We analyzed SARS-CoV-2 outbreaks on cruises in US waters or carrying US citizens and used regression models to compare voyage characteristics. We used compartmental models to simulate the potential impact of 4 interventions (screening for coronavirus disease 2019 (COVID-19) symptoms; viral testing on 2 days and isolation of positive persons; reduction of passengers by 40%, crew by 20%, and reducing port visits to 1) for 7-day and 14-day voyages. RESULTS: During 19 January to 16 April 2020, 89 voyages on 70 ships had known SARS-CoV-2 outbreaks; 16 ships had recurrent outbreaks. There were 1669 reverse transcription polymerase chain reaction (RT-PCR)-confirmed SARS-CoV-2 infections and 29 confirmed deaths. Longer voyages were associated with more cases (adjusted incidence rate ratio, 1.10, 95% confidence interval [CI]: 1.03-1.17, P < .003). Mathematical models showed that 7-day voyages had about 70% fewer cases than 14-day voyages. On 7-day voyages, the most effective interventions were reducing the number of individuals onboard (43.3% reduction in total infections) and testing passengers and crew (42% reduction in total infections). All four interventions reduced transmission by 80.1%, but no single intervention or combination eliminated transmission. Results were similar for 14-day voyages. CONCLUSIONS: SARS-CoV-2 outbreaks on cruises were common during January-April 2020. Despite all interventions modeled, cruise travel still poses a significant SARS-CoV-2 transmission risk.

Acute Gastroenteritis on Cruise Ships - Maritime Illness Database and Reporting System, United States, 2006-2019.

PROBLEM/CONDITION: Gastrointestinal illness is common worldwide and can be transmitted by an infected person or contaminated food, water, or environmental surfaces. Outbreaks of gastrointestinal illness commonly occur in crowded living accommodations or communities where persons are physically close. Pathogens that cause gastrointestinal illness outbreaks can spread quickly in closed and semienclosed environments, such as cruise ships. CDC's Vessel Sanitation Program (VSP) is responsible for conducting public health inspections and monitoring acute gastroenteritis (AGE) illness on cruise ships entering the United States after visiting a foreign port. PERIOD COVERED: 2006-2019. DESCRIPTION OF SYSTEM: VSP maintains the Maritime Illness Database and Reporting System (MIDRS) for monitoring cases of AGE illness among passengers and crew sailing on cruise ships carrying ≥13 passengers and within 15 days of arrival at U.S. ports from foreign ports of call. Cruise ships under VSP jurisdiction are required to submit a standardized report (24-hour report) of AGE case counts for passengers and crew 24-36 hours before arrival at the first U.S. port after traveling internationally. If the cumulative number of AGE cases increases after submission of the 24-hour report, an updated report must be submitted no less than 4 hours before the ship arrives at the U.S. port. A special report is submitted to MIDRS when vessels are within 15 days of arrival at a U.S. port and cumulative case counts reach 2% of the passenger or crew population during a voyage. VSP declares an outbreak when 3% or more of the passengers or crew on a voyage report AGE symptom to the ship's medical staff. RESULTS: During 2006-2019, a total of 37,276 voyage reports from 252 cruise ships were submitted to MIDRS. Of the 252 cruise ships, 80.6% were extra large in size (60,001-120,000 gross registered tons [GRT]), 37.0% and 32.9% had voyages lasting 3-5 days and 8-10 days, respectively, and 53.2% were traveling to a port in the Southeast region of the United States at the time the final MIDRS report was submitted. During 2006-2019, VSP received 18,040 (48.4%) 24-hour routine reports, 18,606 (49.9%) 4-hour update reports, and 612 (1.6%) special reports (2% and 3% AGE reports). Incidence rates decreased from 32.5 cases per 100,000 travel days to 16.9 for passengers and from 13.5 to 5.2 for crew. Among passengers, AGE incidence rates increased with increasing ship size and voyage length. For crew members, rates were significantly higher on extra-large ships (19.8 per 100,000 travel-days) compared with small and large ships and on voyages lasting 6-7 days. Geographically, passenger incidence rates were highest among ships underway to ports in California, Alaska, Texas, New York, Florida, and Louisiana. Among passengers, AGE incidence rates were significantly higher on ships anchoring in California (32.1 per 100,000 travel-days [95% confidence interval (CI) = 31.7-32.4]); among crew, they were significantly higher in the South region of the United States (25.9 [CI = 25.1-26.7]). INTERPRETATION: This report is the first detailed summary of surveillance data from MIDRS during 2006-2019. AGE incidence rates decreased during this time. Incidence rates among passengers were higher on mega and super-mega ships and voyages lasting >7 days. AGE incidence among crew was higher on extra-large ships and voyages lasting 6-7 days. Ship size and voyage length are associated with AGE incidence rates, and more targeted effort is needed to prevent disproportionate AGE incidence rates among passengers and crew sailing in high-risk situations. PUBLIC HEALTH ACTIONS: Maritime AGE surveillance provides important information about the epidemiology of gastrointestinal illness among cruise ship populations traveling in U.S. jurisdictions. AGE illness is highly contagious and can be transmitted quickly within vessels. State and local public health departments in the United States can use data in this report to better inform the traveling public about the risk for AGE and the importance of their role in minimizing the risk for illness while traveling onboard cruise ships. Key elements for reducing exposure to AGE illness, limiting the spread of illness, and preventing AGE outbreaks are proper hand hygiene practices and prompt isolation of symptomatic persons. Passengers can work in collaboration with cruise lines to promote onboard public health by frequently washing their hands, promptly reporting AGE illness symptoms, and isolating themselves from other persons immediately after illness onset. Access to and proper use of handwashing stations can reduce the risk for illness transmission aboard cruise ships.

Pathogenesis and Persistence of Increased Epithelial Mucosubstances in the Nasal Airways of Rats and Mice Episodically Exposed to Ethylene.

Rats repeatedly exposed to high airborne concentrations of ethylene develop eosinophilic rhinitis and mucous cell hyperplasia/hypertrophy (MCH) in nasal respiratory epithelium. Mechanisms underlying these lesions are not well understood to inform occupational exposure guidelines. In this study, we determined (1) the nasal histopathology in rats episodically exposed to ethylene, (2) the ethylene-induced nasal histopathology in similarly exposed mice, and (3) how innate lymphoid cells (ILCs) play a role in ethylene-induced MCH. Animals were exposed to 0 or 10,000 ppm ethylene, 6 h/d, 5 d/wk, for 2 weeks and sacrificed 1 day or 2 weeks postexposure. Others received three 2-week exposure blocks separated by 2-week intervals of no exposure. Episodic exposure was chosen to aid in distinguishing irritant from immune responses. Mucous cell hyperplasia/hypertrophy was induced by ethylene in both species. Rats developed a mild, but transient, eosinophilic rhinitis. Mucous cell hyperplasia/hypertrophy was transient in mice, but persistent in rats. Increases in epithelial mucosubstances after 2 weeks of exposure were only present in ILC-sufficient mice, but not in ILC-deficient mice suggesting that ILCs play a role in MCH and overexpression of genes associated with mucus production/secretion. These findings in animals suggest that inhaled ethylene does not act as a sensitizing agent and will not induce allergen-like nasal airway disease.

CrAssphage as a Novel Tool to Detect Human Fecal Contamination on Environmental Surfaces and Hands.

CrAssphage is a recently discovered human gut-associated bacteriophage. To validate the potential use of crAssphage for detecting human fecal contamination on environmental surfaces and hands, we tested stool samples (n = 60), hand samples (n = 30), and environmental swab samples (n = 201) from 17 norovirus outbreaks for crAssphage by real-time PCR. In addition, we tested stool samples from healthy persons (n = 173), respiratory samples (n = 113), and animal fecal specimens (n = 68) and further sequenced positive samples. Overall, we detected crAssphage in 71.4% of outbreak stool samples, 48%-68.5% of stool samples from healthy persons, 56.2% of environmental swabs, and 60% of hand rinse samples, but not in human respiratory samples or animal fecal samples. CrAssphage sequences could be grouped into 2 major genetic clusters. Our data suggest that crAssphage could be used to detect human fecal contamination on environmental surfaces and hands.

Restaurant Policies and Practices Related to Norovirus Outbreak Size and Duration.

ABSTRACT: Norovirus is the leading cause of foodborne illness outbreaks in the United States, and restaurants are the most common setting of foodborne norovirus outbreaks. Therefore, prevention and control of restaurant-related foodborne norovirus outbreaks is critical to lowering the burden of foodborne illness in the United States. Data for 124 norovirus outbreaks and outbreak restaurants were obtained from Centers for Disease Control and Prevention surveillance systems and analyzed to identify relationships between restaurant characteristics and outbreak size and duration. Findings showed that restaurant characteristics, policies, and practices were linked with both outbreak size and outbreak duration. Compared with their counterparts, restaurants that had smaller outbreaks had the following characteristics: managers received food safety certification, managers and workers received food safety training, food workers wore gloves, and restaurants had cleaning policies. In addition, restaurants that provided food safety training to managers, served food items requiring less complex food preparation, and had fewer managers had shorter outbreaks compared with their counterparts. These findings suggest that restaurant characteristics play a role in norovirus outbreak prevention and intervention; therefore, implementing food safety training, policies, and practices likely reduces norovirus transmission, leading to smaller or shorter outbreaks.

Public Health Responses to COVID-19 Outbreaks on Cruise Ships - Worldwide, February-March 2020.

An estimated 30 million passengers are transported on 272 cruise ships worldwide each year* (1). Cruise ships bring diverse populations into proximity for many days, facilitating transmission of respiratory illness (2). SARS-CoV-2, the virus that causes coronavirus disease (COVID-19) was first identified in Wuhan, China, in December 2019 and has since spread worldwide to at least 187 countries and territories. Widespread COVID-19 transmission on cruise ships has been reported as well (3). Passengers on certain cruise ship voyages might be aged ≥65 years, which places them at greater risk for severe consequences of SARS-CoV-2 infection (4). During February-March 2020, COVID-19 outbreaks associated with three cruise ship voyages have caused more than 800 laboratory-confirmed cases among passengers and crew, including 10 deaths. Transmission occurred across multiple voyages of several ships. This report describes public health responses to COVID-19 outbreaks on these ships. COVID-19 on cruise ships poses a risk for rapid spread of disease, causing outbreaks in a vulnerable population, and aggressive efforts are required to contain spread. All persons should defer all cruise travel worldwide during the COVID-19 pandemic.

A unique mutator phenotype reveals complementary oncogenic lesions leading to acute leukemia.

Mice homozygous for a hypomorphic allele of DNA replication factor minichromosome maintenance protein 2 (designated Mcm2cre/cre) develop precursor T cell lymphoblastic leukemia/lymphoma (pre-T LBL) with 4-32 small interstitial deletions per tumor. Mice that express a NUP98-HOXD13 (NHD13) transgene develop multiple types of leukemia, including myeloid and T and B lymphocyte. All Mcm2cre/cre NHD13+ mice develop pre-T LBL, and 26% develop an unrelated, concurrent B cell precursor acute lymphoblastic leukemia (BCP-ALL). Copy number alteration (CNA) analysis demonstrated that pre-T LBLs were characterized by homozygous deletions of Pten and Tcf3 and partial deletions of Notch1 leading to Notch1 activation. In contrast, BCP-ALLs were characterized by recurrent deletions involving Pax5 and Ptpn1 and copy number gain of Abl1 and Nup214 resulting in a Nup214-Abl1 fusion. We present a model in which Mcm2 deficiency leads to replicative stress, DNA double strand breaks (DSBs), and resultant CNAs due to errors in DNA DSB repair. CNAs that involve critical oncogenic pathways are then selected in vivo as malignant lymphoblasts because of a fitness advantage. Some CNAs, such as those involving Abl1 and Notch1, represent attractive targets for therapy.

Facilitators and Barriers to Conducting Environmental Assessments for Food Establishment Outbreaks, National Environmental Assessment Reporting System, 2014-2016.

Environmental health specialists often perform environmental assessments (EAs) when a suspected or confirmed foodborne illness outbreak is linked to a food establishment. Information from EAs helps officials determine the cause of the outbreak and develop strategies to prevent future outbreaks; however, EAs are not always conducted. To determine facilitators and barriers to conducting EAs, we analyzed open-ended responses reported to the National Environmental Assessment Reporting System about these assessments. We found that EAs were conducted most often when illness was identified, a jurisdiction had a policy to investigate illnesses, and there were resources for such a response. EAs were not conducted in instances such as limited resources, insufficient training, uncooperative facility personnel, or if the establishment fell outside of health department jurisdiction. Identifying the facilitators and barriers to conducting EAs can enable health departments to develop strategies that improve their ability to conduct EAs.

DNA replication stress restricts ribosomal DNA copy number.

Ribosomal RNAs (rRNAs) in budding yeast are encoded by ~100-200 repeats of a 9.1kb sequence arranged in tandem on chromosome XII, the ribosomal DNA (rDNA) locus. Copy number of rDNA repeat units in eukaryotic cells is maintained far in excess of the requirement for ribosome biogenesis. Despite the importance of the repeats for both ribosomal and non-ribosomal functions, it is currently not known how "normal" copy number is determined or maintained. To identify essential genes involved in the maintenance of rDNA copy number, we developed a droplet digital PCR based assay to measure rDNA copy number in yeast and used it to screen a yeast conditional temperature-sensitive mutant collection of essential genes. Our screen revealed that low rDNA copy number is associated with compromised DNA replication. Further, subculturing yeast under two separate conditions of DNA replication stress selected for a contraction of the rDNA array independent of the replication fork blocking protein, Fob1. Interestingly, cells with a contracted array grew better than their counterparts with normal copy number under conditions of DNA replication stress. Our data indicate that DNA replication stresses select for a smaller rDNA array. We speculate that this liberates scarce replication factors for use by the rest of the genome, which in turn helps cells complete DNA replication and continue to propagate. Interestingly, tumors from mini chromosome maintenance 2 (MCM2)-deficient mice also show a loss of rDNA repeats. Our data suggest that a reduction in rDNA copy number may indicate a history of DNA replication stress, and that rDNA array size could serve as a diagnostic marker for replication stress. Taken together, these data begin to suggest the selective pressures that combine to yield a "normal" rDNA copy number.

A Signature of Genomic Instability Resulting from Deficient Replication Licensing.

Insufficient licensing of DNA replication origins has been shown to result in genome instability, stem cell deficiency, and cancers. However, it is unclear whether the DNA damage resulting from deficient replication licensing occurs generally or if specific sites are preferentially affected. To map locations of ongoing DNA damage in vivo, the DNAs present in red blood cell micronuclei were sequenced. Many micronuclei are the product of DNA breaks that leave acentromeric remnants that failed to segregate during mitosis and should reflect the locations of breaks. To validate the approach we show that micronuclear sequences identify known common fragile sites under conditions that induce breaks at these locations (hydroxyurea). In MCM2 deficient mice a different set of preferred breakage sites is identified that includes the tumor suppressor gene Tcf3, which is known to contribute to T-lymphocytic leukemias that arise in these mice, and the 45S rRNA gene repeats.

Acute Gastroenteritis on Cruise Ships - United States, 2008-2014.

From 1990 to 2004, the reported rates of diarrheal disease (three or more loose stools or a greater than normal frequency in a 24-hour period) on cruise ships decreased 2.4%, from 29.2 cases per 100,000 travel days to 28.5 cases (1,2). Increased rates of acute gastroenteritis illness (diarrhea or vomiting that is associated with loose stools, bloody stools, abdominal cramps, headache, muscle aches, or fever) occurred in years that novel strains of norovirus, the most common etiologic agent in cruise ship outbreaks, emerged (3). To determine recent rates of acute gastroenteritis on cruise ships, CDC analyzed combined data for the period 2008-2014 that were submitted by cruise ships sailing in U.S. jurisdiction (defined as passenger vessels carrying ≥13 passengers and within 15 days of arriving in the United States) (4). CDC also reviewed laboratory data to ascertain the causes of acute gastroenteritis outbreaks and examined trends over time. During the study period, the rates of acute gastroenteritis per 100,000 travel days decreased among passengers from 27.2 cases in 2008 to 22.3 in 2014. Rates for crew members remained essentially unchanged (21.3 cases in 2008 and 21.6 in 2014). However, the rate of acute gastroenteritis was significantly higher in 2012 than in 2011 or 2013 for both passengers and crew members, likely related to the emergence of a novel strain of norovirus, GII.4 Sydney (5). During 2008-2014, a total of 133 cruise ship acute gastroenteritis outbreaks were reported, 95 (71%) of which had specimens available for testing. Among these, 92 (97%) were caused by norovirus, and among 80 norovirus specimens for which a genotype was identified, 59 (73.8%) were GII.4 strains. Cruise ship travelers experiencing diarrhea or vomiting should report to the ship medical center promptly so that symptoms can be assessed, proper treatment provided, and control measures implemented.

Effect of minichromosome maintenance protein 2 deficiency on the locations of DNA replication origins.

Minichromosome maintenance (MCM) proteins are loaded onto chromatin during G1-phase and define potential locations of DNA replication initiation. MCM protein deficiency results in genome instability and high rates of cancer in mouse models. Here we develop a method of nascent strand capture and release and show that MCM2 deficiency reduces DNA replication initiation in gene-rich regions of the genome. DNA structural properties are shown to correlate with sequence motifs associated with replication origins and with locations that are preferentially affected by MCM2 deficiency. Reduced nascent strand density correlates with sites of recurrent focal CNVs in tumors arising in MCM2-deficient mice, consistent with a direct relationship between sites of reduced DNA replication initiation and genetic damage. Between 10% and 90% of human tumors, depending on type, carry heterozygous loss or mutation of one or more MCM2-7 genes, which is expected to compromise DNA replication origin licensing and result in elevated rates of genome damage at a subset of gene-rich locations.

Isolation and sequencing of active origins of DNA replication by nascent strand capture and release (NSCR).

Nascent strand capture and release (NSCR) is a method for isolation of short nascent strands to identify origins of DNA replication. The protocol provided involves isolation of total DNA, denaturation, size fractionation on a sucrose gradient, 5'-biotinylation of the appropriate size nucleic acids, binding to a streptavidin coated magnetic beads, intensive washing, and specific release of only the RNA-containing chimeric nascent strand DNA using ribonuclease I (RNase I). The method has been applied to mammalian cells derived from proliferative tissues and cell culture but could be used for any system where DNA replication is primed by a small RNA resulting in chimeric RNA-DNA molecules.

Cdkn1b overexpression in adult mice alters the balance between genome and tissue ageing.

Insufficient cell proliferation has been suggested as a potential cause of age-related tissue dysgenesis in mammals. However, genetic manipulation of cell cycle regulators in the germ lines of mice results in changes in animal size but not progeroid phenotypes. Here we increase levels of the cyclin-dependent kinase inhibitor Cdkn1b (p27kip1) in adult mice through doxycycline-inducible expression and show this results in reduced cell proliferation in multiple tissues. The mice undergo changes resembling ageing even in the absence of an elevated DNA damage response or evidence of senescent cells, suggesting an altered balance between genetic and tissue ageing. In contrast, suppressing cell proliferation by doxycycline treatment of neonates retards growth, but the onset of degenerative changes is delayed during the period of reduced body mass. These results support the hypothesis that many of the most recognizable features of mammalian ageing can result from an imbalance between cell production and the mass of tissue that must be maintained.

Walking associated with public transit: moving toward increased physical activity in the United States.

OBJECTIVES: We assessed changes in transit-associated walking in the United States from 2001 to 2009 and documented their importance to public health. METHODS: We examined transit walk times using the National Household Travel Survey, a telephone survey administered by the US Department of Transportation to examine travel behavior in the United States. RESULTS: People are more likely to transit walk if they are from lower income households, are non-White, and live in large urban areas with access to rail systems. Transit walkers in large urban areas with a rail system were 72% more likely to transit walk 30 minutes or more per day than were those without a rail system. From 2001 to 2009, the estimated number of transit walkers rose from 7.5 million to 9.6 million (a 28% increase); those whose transit-associated walking time was 30 minutes or more increased from approximately 2.6 million to 3.4 million (a 31% increase). CONCLUSIONS: Transit walking contributes to meeting physical activity recommendations. Study results may contribute to transportation-related health impact assessment studies evaluating the impact of proposed transit systems on physical activity, potentially influencing transportation planning decisions.

Post-transcriptional homeostasis and regulation of MCM2-7 in mammalian cells.

The MiniChromosome Maintenance 2-7 (MCM2-7) complex provides essential replicative helicase function. Insufficient MCMs impair the cell cycle and cause genomic instability (GIN), leading to cancer and developmental defects in mice. Remarkably, depletion or mutation of one Mcm can decrease all Mcm levels. Here, we use mice and cells bearing a GIN-causing hypomophic allele of Mcm4 (Chaos3), in conjunction with disruption alleles of other Mcms, to reveal two new mechanisms that regulate MCM protein levels and pre-RC formation. First, the Mcm4(Chaos3) allele, which disrupts MCM4:MCM6 interaction, triggers a Dicer1 and Drosha-dependent ≈ 40% reduction in Mcm2-7 mRNAs. The decreases in Mcm mRNAs coincide with up-regulation of the miR-34 family of microRNAs, which is known to be Trp53-regulated and target Mcms. Second, MCM3 acts as a negative regulator of the MCM2-7 helicase in vivo by complexing with MCM5 in a manner dependent upon a nuclear-export signal-like domain, blocking the recruitment of MCMs onto chromatin. Therefore, the stoichiometry of MCM components and their localization is controlled post-transcriptionally at both the mRNA and protein levels. Alterations to these pathways cause significant defects in cell growth reflected by disease phenotypes in mice.

Ensuring mobility-supporting environments for an aging population: critical actors and collaborations.

Successful aging takes on an array of attributes, including optimal health and community participation. Research indicates that (1) persons with disabilities, including age-related disabilities, report frequent barriers to community participation, including unsuitable building design (43%), transportation (32%), and sidewalks/curbs (31%), and (2) many seniors report an inability to cross roads safely near their homes. This paper attempts to define mobility-related elements that contribute to optimal health and quality of life, within the context of successful aging. It then examines the impacts of community design on individual mobility, delving into which traditional and nontraditional actors-including architects, urban planners, transportation engineers, occupational therapists, and housing authorities-play critical roles in ensuring that community environments serve as facilitators (rather than barriers) to mobility. As America ages, mobility challenges for seniors will only increase unless both traditional aging specialists and many nontraditional actors make a concerted effort to address the challenges.

Cell cycle heterogeneity in the small intestinal crypt and maintenance of genome integrity.

Stem cell quiescence has been hypothesized to suppress the rate at which genetic mutations accumulate within tissues by reducing the number of divisions a cell undergoes. However, recent studies have suggested that stem cells in the small intestine are rapidly dividing. This observation raises the issue of whether replication related errors are an important contributor to the accumulation of genetic damage and, if so, how genomic integrity is maintained within the small intestine. Here, reporter-marked small intestinal epithelial cells, resulting from mini-chromosome maintenance protein 2 (Mcm2) gene driven Cre-mediated recombination, are shown to be retained at the +1 position within the crypt and to contribute to the intestinal epithelia over long periods. Additionally, we show that the rate of cycling of +1 position Mcm2-expressing stem cells is heterogeneous with cycling times ranging between 1 and 4 days. Further, this heterogeneity depends on the p53 signaling pathway and could provide the basis for retention and expansion, through niche succession and crypt fission, of genetically intact stem cells. This somatic selection process would require active cellular replication.