CLOCK and Carcinoma, Lewis Lung: Although we do not have an explanation for these counterintuitive results, we speculate that (1) the proliferation differences between short- and long-period clones is not associated with period differences and that circadian clock–cell cycle coupling may be absent in cancer cell lines such as U-2 OS cells as also reported for Lewis lung carcinoma cells [75] and immortalized rat fibroblasts [76].