Hunter syndrome is an X-linked recessive disorder. Determination of 
heterozygotes is of vital importance in genetic counselling. We describe the DNA 
linkage analysis in 6 Hunter syndrome families and compare it to previous 
results based on a serum assay for IDS activity. Our results confirm the 
reliability of the serum assay. The serum test correctly detected 11/12 of the 
1st degree relatives tested by the serum assay (6/7 carriers and 5/5 
non-carriers). The only case with an apparent false negative result in the serum 
test was a daughter of a "probable heterozygote" whose serum test was also 
negative. We suggest that in this family the mother represented a case of 
germinal mosaicism and her daughter, based on the serum test, was not a carrier. 
If our interpretation is correct, then the apparent false negative results were 
correct. It is concluded that in families where the mutation is not known and 
DNA analysis is not possible due to the lack of informative RFLPs or due to the 
lack of DNA samples on key individuals, as well as in sporadic cases, the serum 
test should be applied as an alternative option for heterozygote detection.