PCDH9 and major depressive disorder: Most of these genes, such as PCDH9, MDD1, MDD2, CREB1 and DISC1, have been identified to be associated with MDD (Cacabelos, Torrellas, & Fernandez‐Novoa, 2016; Xiao et al., 2018), and some of them (e.g. TPH1, GRIN2B and MAOA) were also related to other mental disorders (van Donkelaar et al., 2017; Perlis, 2016; Tovilla‐Zarate et al., 2014).