Schematic expression patterns of sexual development genes. The graphs show possible expression profiles post fertilization in the developing brain (grey line) and testis (black line). (A) Early testis genes (including sex-determining genes) are highly expressed before and/or at the onset of gonadal formation and subsequently down regulated. (B) Late testis genes are expressed later in development, mainly during the formation and maintenance of gonads. (C) Brain genes are higher expressed in the brains than in the testis, forming the brain and/or influencing the sexual development gene network via the action of hormones. Their expression can be maintained (continuous line) or decreased (dashed line) after the first increase in expression.