We have developed a procedure for testing iduronate sulfatase, the enzyme deficient in Hunter syndrome, in single hair roots. Beta-Hexosaminidase was used as the reference enzyme. The ratio of iduronate sulfatase to beta--hexosaminidase, expressed in arbitrary units of activity, is near zero for Hunter patients and greater than 0.6 in almost all roots of normal individuals. Hair roots of Hunter heterozygotes show a characteristic continuum of activity ratios, ranging from totally deficient up to and including the normal range. The results are consistent with the origin of hair roots from a small number of progenitor cells which obey the Lyon hypothesis. The proportion of roots with low activity can be used to discriminate between normal and heterozygous individuals.

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