COX7AR (COX subunit VIIa polypeptide 2-like protein), a novel COX subunit recently found to be involved in mitochondrial supercomplex assembly and respiration, is expressed in high energy-demanding tissues (such as aggressive human BC), responds to estrogen (a nuclear hormone that is the key effector of estrogen-responsive ER-positive breast cancer, one of the three fundamental types of breast cancer), and resembles both COX7A2 and COX7A1 (Zhang et al., 2016). The gene discussed is COX7A2; the disease is breast carcinoma.