Mosaicism for ring and isopseudodicentric chromosome 13.

A three-month-old female infant with multiple malformations was noted on routine cytogenetic evaluation to have dicentric/ring mosaicism of chromosome 13. Additional cytogenic investigations indicated that the dicentric could be further defined as an isopseudodicentric. Unlike the double chromosome break in the more common ring 13 cases, the mechanism for isopseudodicentric/ring generation is attributed to chromosome and chromatid breaks with subsequent bridging, breaking and fusion. The phenotypic features are those of a combined duplication-deficiency of chromosome 13.