COX7RP (cytochrome c oxidase subunit 7a-related polypeptide; also known as COX7A2L/SCAF1), which was originally identified as a protein induced by estrogen in breast cancer cells [22], was found to function as a mitochondrial respiratory chain supercomplex assembly-promoting factor in skeletal muscles derived from Cox7rp-knockout mice [3]. This evidence concerns the gene COX7A2L and breast cancer.