SummaryA 4-month-old Japanese girl with partial monosomy 21 was described. The patient has craniofacial anomalies, a short neck, wide-set nipples, anal atresia, deformed feet, hypertonia, intrauterine growth retardation, and mental deficiency. RFA-and high-resolution GTG-banding chromosome analyses, and Southern-and slot-blot analyses interpreted her karyotype as 45,XX,-2,-21,+der(2)t(2;21)(q37.3;q22.1).The origin of this de novo translocation ascertained by analyses with both QFQheteromorphisms and a Fr8-77/BamHI RFLP was paternal. Comparison of the patient with previously reported patients confirmed that her manifestations are consistent with those of monosomy for 21pter-q21. Based on the results of molecular analyses on the present patient, a DNA clone, Fr8-77 (D21 $82), was assigned to pter-q21.