Giant reduction of InN surface electron accumulation: compensation of surface donors by Mg dopants.

Extreme electron accumulation with sheet density greater than 10(13) cm(-2) is almost universally present at the surface of indium nitride (InN). Here, x-ray photoemission spectroscopy and secondary ion mass spectrometry are used to show that the surface Fermi level decreases as the Mg concentration increases, with the sheet electron density falling to below 10(8) cm(-2). Surface space-charge calculations indicate that the lowering of the surface Fermi level increases the density of unoccupied donor-type surface states and that these are largely compensated by Mg acceptors in the near-surface hole depletion region rather than by accumulated electrons. This is a significant step towards the realization of InN-based optoelectronic devices.

Translational challenges and opportunities in biofilm science: a BRIEF for the future.

Biofilms are increasingly recognised as a critical global issue in a multitude of industries impacting health, food and water security, marine sector, and industrial processes resulting in estimated economic cost of $5 trillion USD annually. A major barrier to the translation of biofilm science is the gap between industrial practices and academic research across the biofilms field. Therefore, there is an urgent need for biofilm research to notice and react to industrially relevant issues to achieve transferable outputs. Regulatory frameworks necessarily bridge gaps between different players, but require a clear, science-driven non-biased underpinning to successfully translate research. Here we introduce a 2-dimensional framework, termed the Biofilm Research-Industrial Engagement Framework (BRIEF) for classifying existing biofilm technologies according to their level of scientific insight, including the understanding of the underlying biofilm system, and their industrial utility accounting for current industrial practices. We evidence the BRIEF with three case studies of biofilm science across healthcare, food & agriculture, and wastewater sectors highlighting the multifaceted issues around the effective translation of biofilm research. Based on these studies, we introduce some advisory guidelines to enhance the translational impact of future research.

Demonstration of Thy-1 antigen on pluripotent hemopoietic stem cells in the rat.

Three approaches were used to demonstrate the presence of Thy-1 antigen on the surface of pluripotent hemopoietic stem cells in the rat. In the first, stem cells from fetal liver, neonatal spleen, and adult bone marrow were prevented from forming hemopoietic colonies in the spleens of irradiated recipients spleen (colony-forming unit assay) by incubation with antibodies to Thy-1 antigen. Highly specific rabbit heteroantiserum to purified rat brain Thy-1 antigen and mouse alloantisera to Thy-1.1-positive thymocytes were equally effective. This inhibition was neutralized by purified Thy-1 antigen. In a second series of experiments, Thy-1-positive and Thy-1-negative populations of nucleated bone marrow cells were separated by the FACS. All of the hemopoietic stem cell activity was recovered in the Thy-1-positive population. The stem cells were among the most strongly positive for Thy-1 antigen, being in the upper 25th percentile for relative fluorescence intensity. The relationships of Thy-1 antigen to the rat bone marrow lymphocyte antigen (BMLA) was shown in a third series of experiments. Rabbit anti-BMLA serum, which is raised against a null population of lymphocyte-like bone marrow cells, has been shown to have anti-stem cell activity. Here we demonstrate by double immunofluorescence, cocapping, and differential absorption studies that Thy-1 and BMLA are parts of the same molecule.

Accurate ultra-low-energy secondary ion mass spectrometry analysis of wide bandgap GaN/In(x)Ga(1-x)N structures using optical conductivity enhancement.

Ultra-low-energy secondary ion mass spectrometry has been used to undertake a structural analysis of GaN-In(x)Ga(1-x)N (x approximately 0.25) quantum wells used in optoelectronic devices. The high resistivity of intrinsic GaN-In(x)Ga(1-x)N restricts the necessary electrical path between the analyzed area and the instrument ground potential resulting in surface charge accumulation. Consequently, unstable and unrepresentative depth profiles tend to be produced. A technique known as optical conductivity enhancement (OCE) has been used during depth profiling to reduce the material resistivity. This creates an electrical path between the sample and holder, eliminating charge build up and resulting in accurate depth profiles.

Hemoglobin Kinetics of the Galapagos Rift Vent Tube Worm Riftia pachyptila Jones (Pogonophora; Vestimentifera).

Kinetics of the reactions of Riftia pachyptila hemoglobin with oxygen were followed spectrophotometrically by stopped-flow and laser flash photolysis techniques. The rate of oxygen dissociation increases eightfold over the range of 5 degrees to 20 degrees C (k = 2.2 sec(-1)at 10 degrees C). Oxygen recombination after flash photolysis was biphasic. The rates of both slow and fast phases of the reaction were independent of temperature from 0 degrees to 20 degrees C(k'fast = 7 x 10(6); k'slow = 1 x 16(6) liter mole (-1) sec(-1)). As the oxygen affinity is relatively temperature independent, analysis in terms of the two-state model of cooperativity requires that the conformational equilibrium constant L decrease by about 50-fold between 3 degrees and 15 degrees C.

Intermittent pneumatic compression - systems and applications.

PRIMARY OBJECTIVE: The purpose of this review is to survey the types of intermittent pneumatic compression systems that are currently used, and their medical applications. MAIN OUTCOMES AND RESULTS: Intermittent compression devices have taken many forms since their initial development, but medical justifications for particular properties of cuff design, compression timing and pressure are often weak. Intermittent compression is well established, and effective in the prevention of deep vein thrombosis (DVT) and reduction of lymphoedema. Other therapeutic applications, such as in chronic arterial and venous disease, are not yet as well accepted, but may become more popular as published evidence increases. CONCLUSIONS: The full potential of intermittent pneumatic compression has probably not yet been realized, and requires better quality research. System design must follow physiological evidence, and while complexity in that design may allow greater therapeutic flexibility, it may incur greater financial cost, difficulty in use, and in the prevention of DVT in particular may be unnecessary.

Wet-chemical etching of atom probe tips for artefact free analyses of nanoscaled semiconductor structures.

We introduce an innovative specimen preparation method employing the selectivity of a wet-chemical etching step to improve data quality and success rates in the atom probe analysis of contemporary semiconductor devices. Firstly, on the example of an SiGe fin embedded in SiO2 we demonstrate how the selective removal of SiO2 from the final APT specimen significantly improves accuracy and reliability of the reconstructed data. With the oxide removal, we eliminate the origin of shape artefacts, i.e. the formation of a non-hemispherical tip shape, that are typically observed in the reconstructed volume of complex systems. Secondly, using the same approach, we increase success rates to ∼90% for the damage-free, 3D site-specific localization of short (250 nm), vertical Si nanowires at the specimen apex. The impact of the abrupt emitter radius change that is introduced by this specimen preparation method is evaluated as being minor using field evaporation simulation and comparison of different reconstruction schemes. The Ge content within the SiGe fin as well as the 3D boron distribution in the Si NW as resolved by atom probe analysis are in good agreement with TEM/EDS and ToF-SIMS analysis, respectively.

Mice lacking the cell adhesion molecule Thy-1 fail to use socially transmitted cues to direct their choice of food.

BACKGROUND: Thy-1 is a major cell-surface glycoprotein of mature neurons and certain other cells, including those of the lymphoreticular system. Despite being the simplest member of the immunoglobulin superfamily, the biological role of Thy-1 has proved elusive. Analysis of Thy-1 null mice has shown the presence of excessive GABAergic inhibition of neurotransmission in the dentate gyrus of the hippocampal formation selectively, without any neurological or behavioural effects being apparent. RESULTS: We show here that Thy-1 null mice are unable to make the appropriate dietary choice in the test for social transmission of food preference, despite showing a normal level of social interaction with the demonstrator mouse, normal neophobia, and normal learning in a T-maze using scented food as cues. The mice also performed normally in tests of anxiety, locomotor activity, exploration of a novel environment, habituation to novelty and spatial learning. This phenotype is maintained on two different strain backgrounds, is rescued by transgenic expression of Thy-1 and by administration of the GABA(A) receptor antagonist pentylenetetrazole. CONCLUSIONS: The test for social transmission of food preference is based on the normal ability of mice in a colony to learn from each other which foods are safe to eat. The lack of this key survival behaviour in Thy-1 null mice could act as an evolutionary pressure point to conserve expression of Thy-1. Furthermore, the specific cognitive defect caused by inactivation of the Thy-1 gene suggests that it would be worthwhile to determine the role of Thy-1 in certain human familial forms of mental retardation that map to chromosome 11q22-23 in the region of the Thy-1 locus rather than the nearby ataxia telangiectasia locus.

Evidence that a slowly cycling subpopulation of adult murine epidermal cells retains carcinogen.

The distribution and persistence of radioactively labeled benzo(a)pyrene [B(a)P] in the skin of adult female SENCAR mice were investigated by autoradiography of epidermal whole mounts and cross-sections at intervals following a single initiating application of 200 nmol of either [3H]B(a)P (2 mCi) or [14C]B(a)P (23 muCi). One day after treatment, the entire thickness of the skin was labeled; the grain density was greatest over hair follicles, sebaceous glands, and interfollicular epidermis. At 1 and 2 weeks, decreases in the nuclear grain density were consistent with the overall pattern of epidermal renewal. One month after treatment, carcinogen label-retaining cells made up approximately 2% of the interfollicular basal cells. They were also present in the hair follicles, approximately 4 and 5% of basal cells in the infundibulum and external root sheath, respectively. They were rare in the germ region and dermal papilla. Carcinogen label-retaining cells were compared with slowly cycling [3H]thymidine label-retaining cells and "maturing" basal cells, two distinct proliferative subsets of adult murine epidermis. Carcinogen label-retaining cells were found to have characteristics of the slowly cycling cells: (a) most of the carcinogen labeled nuclei were found in the central regions of the epidermal proliferative units; (b) treatment of the carcinogen label-retaining cells with 2 micrograms of 12-O-tetradecanoylphorbol-13-acetate elicited labeled mitoses within 1 day, and a general decrease in grain density over basal nuclei. In contrast, maturing basal cells 4 days after a single injection of [3H]thymidine were found at the periphery of the epidermal proliferative units. Within 1 day after treatment with 2 micrograms of 12-O-tetradecanoylphorbol-13-acetate, maturing basal cells were displaced to the suprabasal layers. Double isotope-double emulsion autoradiographs demonstrated doubly labeled cells 1 month after continuous labeling with [3H]thymidine and [14C]B(a)P and provide evidence that the radioactive carcinogen is retained by the slowly cycling [3H]thymidine label-retaining cells. These observations suggest that a slowly cycling population of epidermal cells may be relevant to the initiation phase of two-stage carcinogenesis.

Evidence that the epidermal targets of carcinogen action are found in the interfollicular epidermis of infundibulum as well as in the hair follicles.

Actively cycling, transit-amplifying cells and quiescent cells including stem cells are found in the layer of the epidermis and hair follicles. To determine the origin of skin tumors, we completely removed the interfollicular epidermis of carcinogen-initiated mice by an abrasion technique known to leave the hair follicles undisturbed. The interfollicular epidermis of the abraded mice quickly regenerated from cells in the hair follicles, after which time tumor promotion was begun. Mice in which the interfollicular epidermis had been removed developed papillomas and carcinomas; however, the number of papillomas throughout 40 weeks was half that of the unabraded mice. Carcinoma responses were not significantly different in the abraded and unabraded groups. These results are consistent with the hypothesis that the targets of tumor initiation are stem cells found in the hair follicles and, to a lesser degree, in the interfollicular epidermis.

Distribution of covalent DNA adducts in mouse epidermal subpopulations after topical application of benzo(a)pyrene and 7,12-dimethylbenz(a)anthracene.

The distribution of benzo(a)pyrene [B(a)P] and 7,12-dimethylbenz(a)anthracene (DMBA):DNA adducts was examined in five different subpopulations of SENCAR mouse epidermal cells separated based on buoyant density in continuous gradients of 61.5% Percoll. Three fractions consisted of primarily basal cells (Fractions 3 to 5), while two less dense fractions (Fractions 1 and 2) consisted of primarily differentiating keratinocytes. The levels of B(a)P and DMBA:DNA adducts were examined at 1 h, 6 h, 24 h, 72 h (except DMBA), and 28 days after a single topical application of an initiating dose. Among the basal cell subpopulations, the level of covalent B(a)P:DNA adducts in Fraction 5 cells was significantly higher (P less than 0.05) than Fractions 3 and 4 at every time point examined. On the other hand, B(a)P:DNA adduct levels in Fraction 5 were only significantly higher than Fraction 2 at 6 h and 72 h and not significantly different from Fraction 1 at any time point. With DMBA, no significant differences were initially observed in the levels of covalent DNA adducts among the various Percoll fractions at 1 h and 6 h after treatment. However, at 24 h and at 28 days. Fraction 5 cells had significantly higher (P less than 0.05) levels of covalent DMBA:DNA adducts than Fractions 1 to 4. To explore whether the observed differences in DNA adduct levels were due to differences in metabolic activation, we examined the levels of covalent adducts among epidermal subpopulations after topical application of (+/-)-anti-benzo(a)pyrene-7,8-diol-9,10-epoxide (anti-BPDE). Interestingly, 3 h after treatment with anti-BPDE, significantly higher (P less than 0.05) levels of binding were found in Fraction 5 compared with Fractions 1 to 4. High-pressure liquid chromatographic analyses of B(a)P and DMBA:DNA adducts 6 h and 24 h after treatment did not show any significant differences in adduct profiles among the various subpopulations. These results demonstrate the presence and persistence of hydrocarbon:DNA adducts in all epidermal subpopulations isolated on continuous Percoll gradients for at least 28 days after treatment. Furthermore, of the three basal cell subpopulations, the most dense cells (Fraction 5) developed the highest DNA adduct levels within 24 h and retained these higher levels over 28 days. Finally, differences in DNA adduct levels among epidermal subpopulations do not appear to result from different metabolic capabilities of the cells. The potential significance of these results is discussed in terms of the process of skin tumor initiation.

Quantitation of primary in vitro clonogenic keratinocytes from normal adult murine epidermis, following initiation, and during promotion of epidermal tumors.

A quantitative in vitro assay for clonogenic epidermal cells from adult mice has been designed to focus on the cells from normal epidermis that are potential targets for carcinogens. Dorsal epidermal cells were isolated by trypsinization from groups of three to six CD-1 female mice with yields of 1.31 +/- 6.84 X 10(6) (average of n = 7; SD) viable epidermal cells per square centimeter of skin. Suspensions of single epidermal cells were plated at clonal density onto irradiated Swiss 3T3 cells. The cultures were maintained in high calcium SPRD-105 medium designed to support concomitant proliferation and terminal differentiation of keratinocytes from normal as well as carcinogen-exposed mice. Two weeks later, the dishes were fixed and stained with rhodanile blue, and epidermal colonies were counted. The average number of colonies from normal epidermis was 45 +/- 8.5 (n = 11; SD) per 10(4) cells plated for mice 9 to 69 weeks of age. To determine the effects of initiation on the number of clonogenic epidermal cells, groups of mice 8 weeks of age were treated topically with 200 nmol of 7,12-dimethylbenz(a)anthracene or acetone alone. The number of colonies in both treatment groups remained within the control (untreated) range at all intervals from 7 to 61 weeks after initiation. In contrast, the number of clonogenic cells from control as well as initiated epidermis remained elevated at 1 month following multiple in vivo treatments of skin with 12-O-tetradecanoylphorbol-13-acetate (TPA). The increase in the number of clonogens was always greater from initiated epidermis treated with TPA than from control epidermis treated with TPA. These results suggest that an increase in the clonogenic population was a consequence of promotion rather than initiation and are in agreement with a concomitant carcinogenesis experiment confirming the apparent irreversibility of initiation, the veritable absence of tumors in the absence of promotion, and the similarity of the tumor responses regardless of the age of the animal at initiation or the length of the delay interval between initiation and promotion.

Evidence that cutaneous carcinogen-initiated epithelial cells from mice are quiescent rather than actively cycling.

The basal layer of the epidermis and hair follicles is composed of actively cycling, transit-amplifying cells and quiescent cells including stem cells. To determine which population is the target of carcinogenic chemicals, we treated CD-1 female mice topically with 5-fluorouracil (5-FU), an agent known to kill cycling but not quiescent cells, to probe the origin of the neoplastic lesions. We first determined that 5-FU kills cycling cells in the epidermis. Treatment of mice at 59 days of age (when in anagen 1) with topical 5-FU delayed hair regrowth by 10 days compared to vehicle-treated controls, suggesting that 5-FU killed the cells in anagen. Moreover, 5-FU suppressed the usual hyperplastic response of the epidermal cells to treatment with 12-O-tetradecanoylphorbol-13-acetate. 5-FU reduced the number of epidermal basal cells counted in cross-sections of skin and suppressed DNA synthesis. Approximately 50% of mice treated with 5-FU developed, within 1 week of treatment, a sloughing of the epidermis persisting for 3 weeks, followed by complete healing. Despite the evidence of cell killing in the epidermis and lower hair follicles, in a carcinogenesis experiment where 5-FU or vehicle was applied following tumor initiation with 7,12-dimethylbenz[a]anthracene, the papilloma and carcinoma responses were virtually identical whether or not the mice were treated with 5-FU, suggesting that the tumors arose from quiescent, rather than actively cycling, epidermal cells. When 5-FU was applied before initiation, the papilloma but not the carcinoma responses were slightly but significantly reduced relative to controls. These results are consistent with the hypothesis that the quiescent rather than the rapidly proliferating cells are the targets of tumor initiation.

Both (+/-)syn- and (+/-)anti-7,12-dimethylbenz[a]anthracene-3,4-diol-1,2-epoxides initiate tumors in mouse skin that possess -CAA- to -CTA- mutations at Codon 61 of c-H-ras.

We have determined the tumor-initiating activity of (+/-)syn- and (+/-)anti-7,12-dimethylbenz[a]anthracene-3,4-diol-1,2-epoxide (syn- and anti-DMBADE), the two metabolically formed bay-region diol epoxides of DMBA, and we have also analyzed mutations in the H-ras gene from tumors induced by these compounds. Using a two-stage, initiation-promotion protocol for tumorigenesis in mouse skin, we have found that both syn- and anti-DMBADE are active tumor initiators, and that the occurrence of papillomas is carcinogen dose dependent. All of the papillomas induced by syn-DMBADE (a total of 40 mice), 96% of those induced by anti-DMBADE (a total of 25 mice), and 94% of those induced by DMBA (a total of 16 mice) possessed a -CAA- to -CTA- mutation at codon 61 of H-ras. No mutations in codons 12 or 13 were detected in any tumor. Topical application of syn- and anti-DMBADE produced stable adducts in mouse epidermal DNA, most of which comigrated with stable DNA adducts formed after topical application of DMBA. Further analysis of the data showed that levels of the major syn- and anti-DMBADE-deoxyadenosine adducts formed after topical application of DMBA are sufficient to account for the tumor-initiating activity of this carcinogen on mouse skin. Previously, we showed that both the syn- and anti-DMBADE bind to the adenine (A182) at codon 61 of H-ras. Collectively, these results indicate that the adenine adducts induced by both bay-region diol epoxides of DMBA lead to the mutation at codon 61 of H-ras and, consequently, initiate tumorigenesis in mouse skin.

Ornithine decarboxylase overexpression leads to increased epithelial tumor invasiveness.

Ornithine decarboxylase (ODC) overexpression cooperates with genetic lesions such as an activated c-rasHa to enhance epithelial tumorigenesis. To assess the invasiveness of ODC-overexpressing cells, two noninvasive epidermal cell lines, nontumorigenic BK-1 cells, and the papilloma-derived cell line SP-1 were infected with a replication-defective retrovirus that overexpresses ODC, inoculated into deepithelialized rat tracheas, and transplanted into athymic nude mice. After 5 weeks, ODC-overexpressing BK-1 cells remained localized on the luminal surface of the tracheal xenotransplants, whereas the ODC-overexpressing SP-1 cells were extremely invasive, with the whole tracheal wall penetrated. This invasiveness of ODC-overexpressing SP-1 cells was accompanied by elevated proteinase expression, including increased urokinase plasminogen activator activity in ODC-overexpressing cells and elevated stromelysin-1 mRNA expression in the stromal cells of invaded tracheal transplants.

Si1-x Ge x /Si Interface Profiles Measured to Sub-Nanometer Precision Using uleSIMS Energy Sequencing.

The utility of energy sequencing for extracting an accurate matrix level interface profile using ultra-low energy SIMS (uleSIMS) is reported. Normally incident O2 (+) over an energy range of 0.25-2.5 keV were used to probe the interface between Si0.73Ge0.27/Si, which was also studied using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All the SIMS profiles were linearized by taking the well understood matrix effects on ion yield and erosion rate into account. A method based on simultaneous fitting of the SIMS profiles measured at different energies is presented, which allows the intrinsic sample profile to be determined to sub-nanometer precision. Excellent agreement was found between the directly imaged HAADF-STEM interface and that derived from SIMS. Graphical Abstract ᅟ.

Recombination kinetics following nanosecond laser photolysis of carbonmonoxyhaemogloblin.

The kinetics of the ultrafast ligand recombination following 347 nm laser photolysis of aqueous solutions of carbonmonoxyhaemogloblin have been investigated. The process is biphasic and the rate constants for the two processes as functions of temperature have been used to give activation energies of 6 +/- 3.9 kJ . mol-1 for the fast process and 31 +/- 4.8 kJ . mol-1 for the slow process. Frequency factors have also been calculated. The two processes are discussed in relation to both low-temperature studies and model calculations on the rate of entry of carbon monoxide into haem proteins.

Nanosecond laser photolysis of aqueous carbon monoxy- and oxyhaemoglobin.

The spectra have been measured of the transient species and the final level of absorption observed in nanosecond laser photolysis of aqueous carbon monoxy- and oxyhaemoglobin. These show that the transient absorption change can be interpreted as being due to an ultrafast ligand recombination following the photolysis. The spectra do not support the earlier interpretation (Alpert, B., Banerjee, R. and Lindqvist, L. (1974) Proc. Natl. Acad. Sci. U.S. 71, 558--562) that this was due to a tertiary structural change of the protein.

Comparison of tests of beta-cell function across a range of glucose tolerance from normal to diabetes.

Adequate comparisons of the relative performance of different tests of beta-cell function are not available. We compared discrimination of commonly used in vivo tests of beta-cell function across a range of glucose tolerance in seven subjects with normal glucose tolerance (NGT), eight subjects with impaired glucose tolerance (IGT), and nine subjects with type 2 diabetes. In random order, each subject underwent two of each of the following tests: 1) frequently sampled 0.3-g/kg intravenous glucose tolerance test (FSIVGTT) with MinMod analysis; 2) homeostasis model assessment (HOMA) from three samples at 5-min intervals with a model incorporating immunoreactive or specific insulin measurements; and 3) continuous infusion of 180 mg x min(-1) x m(-2) glucose with model assessment (CIGMA) of three samples at 50, 55, and 60 min (1-h CIGMA) and at 110, 115, and 120 min (2-h CIGMA). The discrimination of each test was assessed by the ratio of the within-subject SD to the underlying between-subject SD, the discriminant ratio (DR). The degree to which tests measured the same physiological variable was assessed using Pearson's correlation coefficient adjusted for attenuation due to test imprecision. An unbiased line of equivalence, taking into account the imprecision of both tests, was used to compare results. Beta-cell function assessed from HOMA and beta-cell function assessed from CIGMA (CIGMA%beta) (using immunoreactive insulin) had higher DRs than first-phase intravenous glucose tolerance test-derived incremental insulin peak, area, insulin-to-glucose index, and acute insulin response to glucose from FSIVGTT-MinMod. CIGMA%beta (immunoreactive insulin) had the highest DR. FSIVGTT-derived first-phase insulin response tests correlated only moderately with HOMA and CIGMA. Using specific rather than immunoreactive insulin for HOMA and CIGMA did not improve discriminatory power. Simple tests such as HOMA and CIGMA, using immunoreactive insulin, offer better beta-cell function discrimination across subjects with NGT, IGT, and type 2 diabetes than measurements derived from FSIVGTT first-phase insulin response.

Genetic regulation of mouse stem cells: identification of two keratinocyte stem cell regulatory loci.

It is well documented that the bulge of hair follicle is a 'niche' for a significant population of mouse keratinocyte stem cells, and 95% of rodent clonogenic keratinocytes originate from the bulge region. The ability to form colonies in vitro is a well recognized test for keratinocyte stem cells. We analyzed the epidermis of seven mouse strains and their segregating crosses [(BALB/c x C57BL/6)F1; (BALB/c x CB6F1); (C57BL/ 6 x CB6F1); (CBF1 x CBF1)F2] for their clonogenic activity in vitro. We found that keratinocyte colony (KC) number is a new quantitative multigenic trait. The analysis of KC size in two parental strains (C57BL/6 and BALB/c), the F1 generation and the segregating crosses demonstrated that the size of KC is a quantitative complex trait also. We determined that mouse epidermis has at least two subpopulations of keratinocytes that gave small (< 2 mm2) and large (> 2 mm2) colonies. The differences in the number of small and large colonies between parental strains (C57BL/6, BALB/c) were significant (P < 0.01). A genome-wide scan of the intercross and the two backcrosses maps the number of small KC to the central region of mouse Chromosome 9 (genomewide P value = 0.01). We define this locus as Ksc1. The proximal region of chromosome 4 is associated with the high number of large KC. We defined this locus as Ksc2. We found that Ksc1 and minor loci on chromosomes 6 and 7 map close, if not equal to, loci associated with mouse skin carcinogenesis. We conclude that mouse epidermis has at least two subpopulations of clonogenic keratinocyte stem cells that are regulated by different genes. We suggest that keratinocyte stem cells responsible for small colonies may play a major role in the regulation of resistance or sensitivity to skin carcinogenesis. Investigation of the genes regulating the stem cell number should provide new insight into the mechanisms of skin carcinogenesis, and should help to develop new approaches for therapies not only against active proliferating tumor cells but also quiescent tumor stem cells.