The demonstration of monozygosity in twins discordant for sacral agenesis.

occurrence. We present here a report of a pair of twins discordant for sacral agenesis. Blood group and HLA determinations and minisatellite DNA probes were used to establish conclusively that these twins were identical. Twin brothers were born at 34 weeks' gestation on 4.1.63. Twin 2 was of lower birth weight at 2069 g and was born with hypospadias and chordee of the penis. He had sacral agenesis with absence of the sacral vertebrae below SI (fig 1), resulting in a neuropathic bladder. He was also found to have a right inguinal hernia and duplication of the right ureter with a ureterocele on the upper pole ureter. He has undergone several surgical repairs of the hypospadias, including more than