NDUFA13 and colorectal carcinoma: Examples include: NDUFA13 (GRIM-19), an essential subunit of complex 1, is reduced in colorectal carcinoma [46]; SDHs, complex 2 subunits, are considered tumor suppressor genes since their mutations lead to tumorigenesis; and COX2, complex 4 subunit, is reduced in many types of cancers ( [24] and ref. therein).