46,XY/47,XYY/48,XYYY karyotype in a 3-year-old boy ascertained because of radioulnar synostosis.

Chromosome analysis was performed on a 3-year-old boy because of bilateral radioulnar synostosis and demonstrated a mosaic karyotype 46,XY/47,XYY/48,XYYY. He had minor facial anomalies and mild intellectual delay. He appears to be the youngest patient reported with this rare chromosome complement. His father, mother, and brother had normal chromosomes. Fluorescence in situ hybridization (FISH) was performed on the propositus and his father with the Y chromosome heterochromatic probe (pHY3.4) to add to the evaluation of mosaicism.