Pallister-Killian syndrome: cytogenetic and molecular studies.

Pallister-Killian syndrome is a dysmorphic syndrome characterized by a tissue-limited mosaicism: a majority of fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. In this study, the interpretation of the extra chromosome as composed of two short arms of chromosome 12 is confirmed, using molecular methods. Furthermore, restriction fragment length polymorphisms indicate that the two arms are identical, which is compatible with the hypothesis of an isochromosome 12p. A new feature which may be important in understanding the mechanism of origin of the abnormality is described: the proportion of abnormal mitoses falls dramatically during long-term culture of fibroblasts.