A female patient with mild mental retardation with spatial perceptual difficulties, microcephaly, depressed nasal root, receding chin, webbed neck, low hairline, shield chest, cubitus valgus, scoliosis and dermatoglyphic findings not characteristic of Down's syndrome is reported. In addition to X/XXX, she had a partial trisomy 21 of the short arm-centromere-proximal long arm segment due to maternal t(12;21) translocation. Two phenotypically normal siblings carried the balanced translocation.