MMP2 and squamous cell carcinoma: As PER1 gene is not only important in regulating cell proliferation, but also in regulating circadian rhythms of living subjects [1], [2], [5], [11], the variation of PER1 expression could not only lead to the circadian changes of oral squamous cell carcinoma cells, but also lead to the variations of MMP-2 expression, PI and MI, by regulating down-streamed cell-cycle-related proteins and tumor-related genes.