PER1 and gastroesophageal reflux disease: Using the cluster correlations from the expression levels of these genes, these gene expression patterns were divided into three related groups, i.e., correlated with the severity in GERD and exhibited rhythmic patterns (PER1, PER2, CRY2, BMAL1, TRPV1 and NGF mRNAs), correlated with the GERD severity but revealed no rhythmic pattern (CRY1, TIM, CB1, GDNF, NHE3 and TAC1 mRNAs), and unrelated to the severity of GERD (CLOCK, PER3, CB2 and NHE1 mRNAs) (Fig. 3A).