The identification of novel mutants and their genetic and phenotypic characterization leads initially to fairly well-defined areas of experimentation. However, some mutant models lend themselves to investigations in fields that at first glance may appear remote from the original observation.

This has been true of the hypogonadal (hpg) mouse first discovered by Bruce Cattanach (Cattanach et al., 1977). In these mutant mice there is a failure of postnatal gonadal development such that paired testicular weight in 60-d-old hpg males is less than 10 mg whilst in normal littermates the testes weigh nearly 200 mg. The seminal vesicles of the mutants are extremely atrophic, indicating a failure of androgen production by the testes. In female mutants ovarian follicles rarely advance beyond the pre-antral stage and the uterus is thin and thread-like.