Also, given that an in vitro study of pancreatic cancer cells has also demonstrated maximum synergy for gemcitabine when exposure to a thymidylate synthase inhibitor such as 5-FU precedes exposure to gemcitabine (Rauchwerger et al, 2000), we adopted the regimen of gemcitabine administration on days 8 and 15 after S-1 administration of each cycle. This evidence concerns the gene TYMS and familial pancreatic carcinoma.