ABCB1 and metastatic colorectal cancer: ABCB1 c.1236 C allele was significantly associated with grade 3/4 toxicities in metastatic colorectal cancer patients12, and ABCB1 c.1236 T/T genotype was also associated with significantly increased exposure to irinotecan and its active metabolite SN-38 compared to those with heterozygous and wild-type13.