CDK5 and psychiatric disorder: Because PER2 is involved in several physiologically relevant pathways in addition to clock regulation (Albrecht et al., 2007), PER2 may mediate several biological functions that were previously linked to CDK5, such as the regulation of the brain reward system (Benavides et al., 2007; Bibb et al., 2001) and psychiatric diseases (Engmann et al., 2011; Zhu et al., 2012).