In vivo hprt mutant frequencies in T-cells of normal human newborns.

Mutation at the hypoxanthine-guanine phosphoribosyl transferase locus (hprt; HPRT enzyme) in the human fetus was studied by clonal assay of placental cord blood samples from full-term newborns. Conditions for determining hprt mutant frequencies, as defined for adults, were also optimal for studies in newborns. The mean mutant frequency for 45 normal human newborns (37 male, 8 female) was 0.64 X 10(-6) (SD = 0.41 X 10(-6); median value = 0.58 X 10(-6). These values are approx. 10-fold lower than corresponding adult hprt mutant frequency values. Factors such as limiting-dilution cloning efficiencies, delay prior to study of sample, sex, cryopreservation or technician performing the assay did not significantly affect assay results. Maternal smoking did not result in elevated mutant frequency values. Most wild-type and mutant clones studied were CD4 surface antigen positive (helper/inducer). All hprt mutants analyzed lacked HPRT activity.