Bruce Cattanach has made important contributions to the genetics of mammalian sex determination and differentiation, X-chromosome inactivation, chromosome imprinting and experimental mutagenesis. Bruce’s X-autosome translocation, which he discovered and investigated, has been used worldwide in studies of X-chromosome action. With it he showed that the phenomenon of position effect variegation is due to variable reactivation of previously inactivated loci. It gave him the first evidence of the Xce locus, which affects the choice of X for inactivation, and probably represents the inactivation centre. He discovered and studied the sex-reversed mutation, Sxr, which has provided important information on the roles of the X and Y chromosomes in germ cells, and evidence of meiotic crossing-over between X and Y. Another mutant, hypogonadal (hpg), deficient in gonadotrophin-releasing factor, has been a valuable tool in studies of the hypothalamic gonadotrophin-releasing system. A recent finding is a chromosome imprinting effect, leading to complementary phenotypes in mice with maternal or paternal disomy for particular chromosomes. He was the first to study chemical mutagenesis in the mouse.