The open reading frame of the SRY gene has been examined in a series of 22 XY females with clinically defined pure gonadal dysgenesis by direct sequencing of biotinylated PCR product bound to streptavidin coated beads. Amongst the 22 XY females examined, five (two of whom are sisters) were found to have single base changes all within the highly conserved DNA binding (or HMG box) domain. In the remaining 17 cases, the SRY gene sequence was indistinguishable from that found in normal males. In three of the XY females with point mutations, the altered amino acids occur in highly conserved positions leading to non-conservative changes (Arg to Gly at position 5, Met to Thr at position 21 and Arg to Trp at position 76). Examination of the SRY gene from the father's Y chromosome has shown that the mutations at position 5 in patient SHM60 and position 21 in patient HN31 have arisen de novo. In the case of the two sibs, both have identical mutations where a C to T transition in codon 17 has created a TAG termination signal, thus suggesting that the deceased father is likely to be a gonadal mosaic for the mutation. In the case of the mutations at positions 17 and 76, the fathers are not available for investigation and so it has not been possible to determine whether the changes are de novo. These data indicate that the majority of XY females with pure gonadal dysgenesis owe their sex-reversed phenotype to mutations in as yet uncharacterised segments of the SRY gene, or, at other loci acting early in the sex-determining pathway.