Monosomy 21: a possible stepwise evolution of the karyotype.

We describe a female infant with manifestations of complete monosomy for chromosome 21 intrauterine growth retardation, failure to thrive, craniofacial anomalies, arthrogryposis-like features, and psychomotor retardation. Chromosome analysis demonstrated mosaicism for three different cell lines in the various tissues examined; 45,XX,-21/46,XX,del(21)(q11)46,XX. The existence of these three lines suggests a possible explanation for the few cases of "complete monosomy 21" which have been reported.