Discordant Hirschsprung's disease in monozygotic twins

Two pairs of twins are being reported where monozygocity was supported by the observed sharing of a single and common placenta and by ABO and HLA identity. Only one of each pair of twins was affected by long-segment Hirschsprung's disease; the other twins were entirely normal. Discordant Hirschsprung's disease in monozygotic twins is very rare. The etiology in such cases is influenced by genetic or by environmental intrauterine factors rather than by postnatal influences, as suggested elsewhere.