Culture colony of the bacteriophage M13, used as a source of DNA fragments containing the S^Iry^i (sex- determining region Y) gene, the switch that redirects mammalian development from its usual pathway - female - to male. Joint research by the Medical Research Council & Imperial Cancer Research Fund showed that some normal, chromosomally-female mice embryos developed as males with testes following their injection with a small 14-kilobase fragment of DNA containing S^Iry^i. The DNA for S^Iry^i was released from its phage vector by digestion with a restriction enzyme, isolated by gel electrophoresis (part of the autoradiogram is shown here) & then purified.