Unbalanced 18q/21q translocation in a patient previously reported as monosomy 21.

[monosomy 21]

We describe a patient in whom full monosomy 21 was initially assumed from routine GTG-banded karyotyping . Re-examination with chromosome painting demonstrated an unbalanced translocation between the long arms of chromosomes 18 and 21 . Fluorescence in situ hybridisation (FISH ) and microsatellite marker analysis revealed partial monosomy of chromosome 21 (pter-q 21 ) and 18 (q 22 -qter ). The patient , 18 years old at the second examination , revealed multiple dysmorphic features , genital hypoplasia , dilated cerebral ventricles , muscular hypotonia and severe mental retardation . In not one out of all patients investigated postnatally in whom an initial examination had revealed monosomy 21 , this could be confirmed by FISH ; in all of them , re -examination detected an unbalanced rearrangement leading to only partial monosomy 21 plus partial monosomy of another chromosome to which the distal 21 q segment was attached . Thus , it is still highly likely that full monosomy 21 is incompatible with intra-uterine survival .