We further compared the DEGs with 5-fluorouracil (5-FU) resistance-related genes in GC and CRC and found that melatonin might downregulate the expression levels of 5-FU resistance-related genes, such as thymidylate synthase (TS) in GC patients and ATR, CHEK1, BAX and MYC in CRC patients. Here, CHEK1 is linked to colorectal carcinoma.