Several attempts have been made to correlate clock gene polymorphisms or expression levels in either cancer cell lines or tissues from various cancers to determine whether clock gene mutations (62) or levels of expression (63, 64, 65, 66, 67, 68, 69, 70) play a role in initiation or progression of cancer and susceptibility of cancer to a particular drug and a particular time of day for delivery of the drug (chronochemotherapy). This evidence concerns the gene CLOCK and cancer.