Other presentations to this symposium have indicated that the search at the molecular level for the pivotal regulatory, or structural, gene responsible for determining the development of the undifferentiated gonad has been joined in earnest. It is also clear that genes on the Y chromosome are involved in processes other than primary determination of the testis. In this summary, we will review briefly 'the molecular search for the sex-determining gene' and consider the approaches that are available and the achievements that have been made in the areas relevant to an understanding of the roles and significance of other Y-located genes. The availability of molecular and physical mapping data also allow an examination of the evolutionary relationship of the mammalian X and Y chromosomes and a consideration of the possible homologies between the human and mouse Y chromosomes.