PCDH9 and major depressive disorder: Among the 320 depression-associated genes we tested, seven genes were under positive selection in the human lineage (Table 1, Fig. 1), including the immune response-related gene STAU1 (Pang et al. 2021), the neurodegeneration-related gene PSEN2 (Fedeli et al. 2019), the neurological-related gene ANKK1 (Hoenicka et al. 2010), the electron transfer flavoprotein dehydrogenase ETFDH (Zhang and Zhao 2022), zinc fingers and homeoboxes 3 ZHX3, the neural development-related gene PCDH9 (Chen et al. 2024), and immune-related gene LYRM4 (Wang et al. 2023).