A mentally retarded boy was found to have an unusual chromosomal mosai
cism [46,XY, del(18) (q22)/46,XY,iso psu dic(18)(q23)]. The clinical m
anifestations are compatible with the 18q- syndrome, The chromosome al
teration was defined by high resolution banding and fluorescence in si
tu hybridization (FISH). A mechanism to explain the origin of the two
cell lines is presented and discussed. (C) 1994 Wiley-Liss, Inc.