In the Thai population, treated with irinotecan for colorectal cancer, genotyped for ABCB1 c.1236C > T, ABCB1 c.3435C > T, ABCC2 c.3972C > T, ABCG2 c.421C > A, CYP3A4*1B, CYP3A4*18, CYP3A5*3, DPYD*5, UGT1A1*28, and UGT1A1*6 variants, ABCC2 c.3972C > T was associated with grade 1–4 neutropenia; and patients carrying both UGT1A1*28 and *6 were associated with severe neutropenia [131]. Here, ABCB1 is linked to colorectal cancer.