PER2 and neoplasm: In cancer cells, both PER1 and PER2 were shown to act as tumor suppressors (Fu et al., 2002, 2016; Yang et al., 2009; Su et al., 2017; Zhu et al., 2019; Hou et al., 2020; Liu et al., 2020) and mutations in these genes were associated with human cancer (Chen et al., 2005; Kettner et al., 2014; Wu et al., 2019).