Tissue-specific mosaicism for the stability of a ring 13 chromosome.

A ring 13 chromosome was identified in an infant whose chromosomes were studied because of microcephaly. The ring chromosome was studied over a 3-year-period in lymphocytes, and in both short- and long-term fibroblast cultures. Lymphocyte cultures revealed a consistently stable ring 13 chromosome with minimal loss of genetic material (the distal portion of band q34). Fibroblast cultures contained a ring chromosome a quarter of the size of the original ring and this chromosome was unstable in short- and long-term cultures. The patient's mild dysmorphic features and moderate mental retardation correlate with a stable ring chromosome in which only a small amount of genetic material has been lost rather than with the unstable small ring 13 chromosome observed in fibroblast cultures. The observation of drastic tissue specific differences in ring sizes and stability makes phenotypic karyotypic correlations with ring chromosome patients even more difficult and counselling in cases of prenatal diagnosis questionable.