CYP2A6 and colorectal neoplasm: Moreover, although the mechanism is unknown, chemoprevention using daily aspirin to reduce the risk the colorectal tumors was found to be inversely dependent on the putative enzyme activity of the CYP2A6 phenotype (based on the presence/absence of CYP2A6*1 alleles) among a Japanese cohort without familial adenomatous polyposis (Fig. 1e, f), especially in nonsmoking men (Table 1).