NDUFA13 and cancer: For example, NDUFA13 (GRIM-19), an essential subunit of complex I, is downregulated in colon carcinogenesis (47); mutations in SDHs–complex II subunits, lead to tumorigenesis, and they are therefore considered to be tumor-suppressor genes; similarly, the expression level of COX2–complex IV subunit 2, is reduced in many types of cancers (48).