CTCF and breast cancer: To investigate whether CTCF loops are enriched for enhancers relative to surrounding regions, we investigated the enhancer-associated H3K4me1 (histone 3 lysine 4 monomethylation) as well as other chromatin marks within and around experimentally determined CTCF loops determined using the ChIA-PET technique in K562 myeloid leukemia and MCF-7 breast cancer cell lines.