UQCRB (ubiquinol-cytochrome c reductase binding protein), also known as complex III subunit VII, ubiquinol-cytochrome c reductase complex 14 kDa protein and QCR7, is a 111 amino acid mitochondrion inner membrane protein. UQCRB is a component of the ubiquinol-cytochrome c reductase complex, also known as complex III or cytochrome b-c1 complex, which is a constituent of the mitochondrial respiratory chain. The ubiquinol-cytochrome c reductase complex is composed of two core proteins (UQCRC1 and UQCRC2), three respiratory subunits (cytochrome c1, cytochrome b and Rieske) and six low-molecular weight proteins (UQCRH, UQCRB, QP-C, UQCR10, UQCR11 and a cleavage product of Rieske). UQCRB binds ubiquinone and is involved in the transfer of electrons across the mitochondrion inner membrane. Mutations in the gene that encodes UQCRB have been linked to mitochondrial complex III deficiency (CIII deficiency), a condition characterized by congenital lactic acidosis.