Loss-of-function and gain-of-function studies have revealed that several transcription factors and growth factors are implicated in gonad differentiation. In fact, the mice in which certain genes encoding transcription factors or growth factors are disrupted or overexpressed showed a variable defects in gonad differentiation and gonad sex determination. These phenotypes together with the symptoms of the corresponding human diseases indicated the functional significance of the genes. Although these studies largely contributed to identify the components essential for the gonad differentiation and gonad sex determination, functional relation among the components remained to be elucidated. Elucidation of the genetic cascade will help us to understand the molecular mechanisms underlying gonad differentiation and gonad sex differentiation.