Sex reversal in mice: a chromosomally-female (XX) mouse carrying the Sry-gene (sex-determining region Y gene) that has caused it to grow as a male with testes. This mouse is a product of research led jointly by Robin Lovell-Badge of the Medical Research Council & Peter Goodfellow of the Imperial Cancer Research Fund. A number of embryos were injected with a small fragment of Y chromosome DNA containing the Sry-gene; sex reversal occurred in some of the females. Sry is believed to act as a switch in the complex gene mechanism that diverts mammalian development from its usual course - the female - to the male.