In this study, the expression profiles of eight circadian clock genes (PER1, PER2, PER3, CRY1, CRY2, CKIε, CLOCK, and BMAL1) of cancerous and noncancerous tissues from 29 GC patients were investigated using real-time quantitative reverse-transcriptase polymerase chain reaction and validated through immunohistochemical analysis. The gene discussed is PER2; the disease is gastric cancer.