Sex reversal in mice: at right is 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. At left is a normal male. The mice are products of joint research by the Medical Research Council and 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.