A total of 153 hprt mutants (23 spontaneous, 130 gamma-ray-induced) of primary human skin fibroblasts were isolated and genetic alterations at the locus were studied by multiplex polymerase chain reaction (PCR). The analyses showed that 51% (66/130) of gamma-ray-induced genetic changes were large deletions, whereas the majority of spontaneous mutants (21/23) exhibited point mutations. The spectrum of large genetic alterations appeared to be dependent on dose in gamma-ray-induced (1-4 Gy) mutations; mutants with complete loss of the hprt locus comprised 21 (3/14) or 39% (15/38) of clones isolated after irradiation with 1 or 4 Gy, respectively. The frequency of partial deletions was found to be higher in the mutants isolated from clones irradiated with 2 Gy (38%) than from those irradiated with 4 Gy (8%). Mapping of all intragenic deletion breakpoints exhibited a nonrandom distribution of breakpoints toward the 3' end of the hprt gene.