[Tetrasomy of the short arm of human chromosome 18: cytogenetics and phenotypic disorders].

Two mentally retarded girls with a small metacentric nonsatellite extrachromosome were examined. Probands were found to share many clinical features: asthenic constitution, microcephalia, low-set malformed ears, high arched palate, long fingers and toes, a wide gap between first and second toes, clinodactyly of the 1st and 5th fingers, scoliosis. The extrachromosome was unequivocally interpreted as an isochromosome for the short arm of chromosome 18.