SLC22A4 and major depressive disorder: Especially, the genes in Class 1 tend to be more highly expressed in females than males, which may be associated with the higher incidence rate of depression in female patients.42,43 The solute carrier Family 24 Member 2 (SLC24A2) in Class 1 was found to have the most expression differences between males and females (P = 1.19 × 10−4), as shown in Fig. 7B.