The molecular basis of somatic mutation at the hypoxanthine-guanine phosphoribosyl-transferase (hprt) locus in human 6-thioguanine resistant T-cell clones from 17 individuals has been studied by Southern blot analysis, multiplex PCR (polymerase chain reaction) and direct sequencing of PCR amplified hprt cDNAs or genomic DNA. Twenty-three novel mutations were detected, which in addition to previously described mutations provide a background mutational spectrum based on a total of 45 hprt mutations in human T-cells. Twenty T-cell mutants had base substitutions in the coding region leading to 15 missense and five nonsense mutations. In addition to five frameshift mutations caused by four small deletions and one duplication, seven splice mutations, three of them with skipping of exon 8, were detected. Thirteen genomic structural alterations have also been identified; one of these had a genomic exon 1 deletion with a GGCCGG-hexamer in both breakpoints.