PER2 and colorectal adenocarcinoma: For example, Caco-2 cells are a commonly studied human colorectal adenocarcinoma cell line that has recently been shown to exhibit circadian oscillations of circadian component PERIOD2 (PER2) (Ballesta et al., 2011; Swanson et al., 2011); however, the limited capacity of Caco-2 cells for differentiation and their dampened circadian oscillations over time might hinder extrapolation of in vitro results to functional GI tissue (Hughes et al., 2009).