Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH) is is a 146 kDa tetramer metabolic enzyme primarily responsible for catalyzing the reversible oxidative phosphorylation of glyceraldehyde 3-phosphate in the glycolytic pathway. Apart from a role in glycolysis, GAPDH may have other roles such as in the activation of transcription.

GAPDH is reported to bind to a variety of other proteins, including the amyloid precursor protein (mutations in which cause some forms of Alzheimer's disease) and the polyglutamine tracts of Huntingtin (the protein product, aberrant forms of which are causative of Huntington's disease). Associations with actin and tubulin have also been reported. GAPDH protein may also have a role in the regulation of apoptosis, and interestingly migrates from the cytoplasm into the nucleus when cells become apoptotic.

GAPDH is considered a "house keeping gene" because it is expressed in large amounts and is required at all times for an array of important cell maintainece functions. Therefore, GAPDH mRNA levels are often used as standards in studies of mRNA expression. Additionally, scientists may use specific antibodies to GAPDH as loading controls for western blot experiments.