In mouse sex determination, the presence or absence of Sertoli cells in the developing gonad is essential for the decision to form either a testis or an ovary. The transcription factor SOX9 has emerged as the master regulator of Sertoli cell differentiation during testis development and thus the crucial gene to determine sex. It is the target of two sets of regulatory controls, one positive and one negative, where one set tries to gain dominance over the other in the early gonad and then to establish and maintain the activity or silence of Sox9 throughout life. The data reveal the importance of the positive regulatory loops to reinforce initial decisions, whereas the maintenance of the gonadal phenotype appears to rely on the active repression of the opposite pathway.