ALK and major depressive disorder: ALK, a receptor tyrosine kinase, has previously been implicated in a number of psychiatric conditions – schizophrenia, depression, anxiety, and AUDs/SUDs (Kunugi et al., 2006; Bilsland et al., 2008; Lasek et al., 2011b; Wang et al., 2011; Weiss et al., 2012; He et al., 2015; Schweitzer et al., 2016; Dutton et al., 2017).