Blood samples are collected at baseline to assess the influence of polymorphisms (CYP2C9, uridine diphosphate (UDP)-glucuronosyl transferase, A6, glutathione S-transferase [GST] M1, and Glutathione S-transferase (GST) theta 1 (GSTT1)) on age of onset of phenotype or number of colorectal polyps. Plasma drug trough levels are assessed at baseline, 1 month, and 3 months.