AICDA and cancer: To evaluate STAG1 as synthetic-lethal dependency that is relevant in a wide range of STAG2-deficient cancers (Figure 2—figure supplement 3A; Bailey et al., 2021; van der Lelij et al., 2020; Benedetti et al., 2014; van der Lelij et al., 2017), we engineered an isogenic pair of STAG2-wildtype and -deficient HCT 116 colon carcinoma cells and homozygously inserted ARTi target sites (besides an AID::V5 tag; explained in the ‘Materials and methods’ section) into the 3′-UTR of endogenous STAG1 (Figure 2I).