Relations between sex and genes are well understood in regards to the pathways that control sex differentiation and leading to phenotypic sex. However, the question of the role of genes in gender identity, sexual and affective orientation, or sexual function remains elusive. If the role of steroid hormones on brain development is established, their role on gender identity or sexual orientation is still hypothetical. Concerning sexual function, the attempts to define which genes are potentially involved, have only pointed to genes associated to neurotransmission, but no breakthrough discovery has been made.