PER1 and cancer: The system is further complicated by Casein kinase 1 ε that serves as a mediator of PER at a posttranslational level and makes the proteins aberrantly expressed via phosphorylation; significantly lower expression of the PER3 gene has been detected in cancer tissue relative to the adjacent normal mucosa.26 There is abundant evidence supporting a causative connection between degradation of PER3 and human carcinogenesis.