While there are multiple human glyceraldehyde 3-phosphate dehydrogenase-like pseudogenes, there is only one glyceraldehyde 3-phosphate dehydrogenase gene expressed in somatic tissue (Benham and Povey 1989). Consistent with this conclusion, the homogeneous enzymes purified from various human tissues had indistinguishable physical and immunochemical properties (Heinz and Freimuller 1982), and studies of human erythrocytes of various ages suggested that variant forms of the enzyme arise as a result of post-translational modifications (Edwards et al. 1976). There is, however, an authentic second isoform of glyceraldehyde 3-phosphate dehydrogenase whose expression is confined to spermatogenic cells of the testis (Welch et al. 2000).