Downregulations of PER1, PER2, or PER3 have been observed in various cancers, including oral squamous cell carcinoma (Gong et al., 2021), head and neck squamous cell carcinoma (Li et al., 2019; Rahman et al., 2019), colorectal cancer (Orhan et al., 2019; Sahar et al., 2022), breast cancer (Liu et al., 2021; Liu et al., 2022a), ovarian cancer (Angelousi et al., 2019; Chen et al., 2021), melanoma (Lesicka et al., 2023), and hematological malignancies (Jiang et al., 2021). This evidence concerns the gene PER2 and breast cancer.