Trisomy 17 mosaicism in amniotic fluid cells not found at birth in blood but present in skin fibroblasts

This article describes a case of fetal trisomy 17 mosaicism found in amniot ic fluid cells in one of two bichorial biamniotic twins without any sonogra phic anomaly. The extra chromosome 17 was absent from cord blood cells at b irth but present on karyotype and in situ hybridization in cultured fibrobl asts from skin biopsy. Clinical examination showed a few mild dysmorphic fe atures and a moderate neurological involvement which may rather be related to prematurity. It therefore seemed important to obtain the karyotype on fi broblasts when a trisomic cell line was found in amniocentesis and not conf irmed on blood lymphocytes, even in the absence of dysmorphic features. Thi s should help to differentiate a real mosaic from a mosaic restricted to ex tra-fetal tissues. Copyright (C) 1999 John Wiley & Sons, Ltd.