In our study, the induction of resistance to irinotecan in two colon cancer cell lines (HCT116 and SW480) resulted in increased expression of ABCG2, CYP3A4, and MCL1. At the same time, MLH1 expression also increased in resistant cells derived from HCT116. The gene discussed is CYP3A4; the disease is malignant colon neoplasm.