To circumvent this, AAV-mediated human somatic cell gene targeting was used to “knock-in” (KI) tumor-derived mutations into the endogenous allele of STAG2 in HCT116 cells, a human cancer cell line with a single, wild-type allele of STAG2, intact sister chromatid cohesion, and a near-diploid karyotype. The gene discussed is STAG2; the disease is neoplasm.