Using CRISPR/Cas9 genome engineering, we generated HCT 116 colon carcinoma cells that stably integrated a GFP-tagged AID (Morawska & Ulrich, 2013) at the N terminus of both alleles of the endogenous STAG1 gene in the parental STAG2–wild-type cells and an isogenic HCT 116 cell line carrying a previously introduced STAG2 T220fs null mutation (STAG2-502c4, described in van der Lelij et al [2017]) (Fig 2A). The gene discussed is AICDA; the disease is colon carcinoma.