Mutations in the X chromosome hypoxanthine-guanine phosphoribosyl transferase 
(HPRT) gene are responsible for Lesch-Nyhan syndrome and related diseases in 
humans. Because the gene is on the X chromosome, males are affected and females 
in the families are at risk of being carriers of the mutation. Because there are 
so many different mutations that can cause the disease (218 different mutations 
in 271 families), genetic testing for carrier status of females requires 
detailed molecular analysis of the familial mutation. This analysis can be 
complicated by the unavailability of an affected male for study. In addition, 
when the mutation is a deletion (34 reported instances), molecular analysis in 
females is difficult because of the two X chromosomes. We have applied a 
peripheral blood T lymphocyte cloning assay that uses resistance to the purine 
analogue 6-thioguanine (TG) to measure the frequency of cells in females 
expressing a mutant HPRT allele to determine mutation carrier status in 123 
females in 61 families. In families in which the HPRT mutation was determined 
and could be easily analyzed in samples from females, we found a mean (+/- SD) 
mutant frequency of 9.7 (+/- 8.7) x 10(-6) in noncarrier females and 2.9 (+/- 
3.0) x 10(-2) in carrier females. The frequency in carrier females is less than 
the 0.5 expected for nonrandom X inactivation because of in vivo selection 
against HPRT mutation-expressing T lymphocytes or stem cells during prenatal 
development. The use of this cloning assay allows determination of the carrier 
status of females even when the HPRT mutation is not yet known or is difficult 
to determine in DNA samples from females. This approach provides a rapid assay 
that yields information on carrier status within 10 days of sample receipt.