CTCF and breast cancer: Next, to study the effect of altered CTCF expression in mammary tissue independently of the heavy mutational burden of breast cancer models, we investigated the effect of low CTCF expression in the nontransformed breast epithelial cell line MCF10A carrying a single allele KO of CTCF (MCF10A-CTCF+/−) via CRISPR-Cas9 (21) (Fig. 1D).