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.