A 2.5-year-old girl who presented with abdominal distension, hepatomegaly, 
coarse facies, hirsutism and contraction deformities was investigated for 
mucopolysaccharidoses. Urinary excretion showed increased total 
glycosaminoglycans (105 mg/mmol creatinine; normal for age 9-20 mg/mmol) with 
marked increases of dermatan and heparan sulphates. A number of lysosomal enzyme 
activities were measured on leucocytes, serum and cultured fibroblasts. Normal 
or high activities were found for alpha-iduronidase, 
N-acetylgalactosamine-6-sulphatase, beta-galactosidase, arylsulphatase B and 
beta-glucuronidase. However a marked deficiency of iduronate sulphate sulphatase 
activity was observed, consistent with a diagnosis of Hunter's disease. 
Activities were reduced to less than 2% of mean control values in the patient's 
leucocytes, serum and cultured fibroblasts. Normal activities were measured in 
samples from the father and younger sister but a partial deficiency (43% of 
control serum) was found in the mother. Chromosome studies on the patient 
revealed a partial deletion of the long arm of one X-chromosome, most probably 
of band Xq25, which was not inherited from either parent. Studies using BrdU 
indicated that the deleted X chromosome was consistently late replicating, and 
as a result the Hunter gene was fully expressed on the other X chromosome.