The majority of studied CpG were found to be methylated in ER+ but not in ER− BC samples, a finding that was further corroborated in vitro by a MassARRAY analysis of IDO1 promoter methylation patterns in ER+ BC cell lines (namely MCF-7, ZR-75-1, and BT-474), which contrasted with ER− MDA-MB-231 BC cells [71]. The gene discussed is IDO1; the disease is breast cancer.