Female twin with hunter disease due to nonrandom inactivation of the X‐chromosome: A consequence of twinning

Abstract
We report the occurrence of Hunter disease (mucopolysaccharidosis type II) in a karyotypically normal girl who was one of identical twins. Molecular studies showed nonrandom X‐inactivation in both her fibroblasts and lymphocytes, while her normal twin showed equal usage of both X chromosomes. In view of previous reports of 7 pairs of identical female twins in which one had Duchenne muscular dystrophy, it seems that twinning may be strongly associated with nonrandom X‐inactivation, and is not specific to the properties of the disease causing gene.