GAPDH is a 146kD tetramer glycolytic pathway metabolic enzyme composed of four 30-40 kDa subunits. It is responsible for reversibly phosphorylating its substrate glyceraldehyde 3-phosphate within the glycolytic pathway. Apart from its role in glycolysis, GAPDH may have other roles such as transcriptional activation. Due to its housekeeping role, GAPDH is highly expressed in almost all tissues, allowing its use as an internal loading control (traditionally for mRNA expression comparisons, but also in protein studies.