Three new cases of partial monosomy 21 resulting from one ring 21 chromosome and two unbalanced reciprocal translocations.

Three patients with partial monosomy of the long arm of chromosome 21 are reported. Each one presents several features of a 21q--syndrome but in cases 2 and 3, other chromosomes are involved, contributing to the variability of the clinical picture. Synthesis of clinical, enzymatic and cytogenetic findings confirms that the superoxide dismutase A (SOD-A) locus is in sub-band 21q22-1. However, it is not possible to localize precisely the segments responsible for the different clinical features of 21q--syndrome.