GAP43 is an abundant protein which is found heavily concentrated in developing neurons, in particular at the growing tips, the growth cones. One group discovered it since it becomes unregulated during the regeneration of the toad optic nerve, and named it “growth associated protein 43”, the 43 referring to the apparent molecular weight on SDS-PAGE gels (1). GAP43 does not run on SDS-PAGE in a fashion which accurately reflects its molecular weight, since the full length molecule human GAP43 is 238 amino acids giving a real molecular weight 24.8kDa. The molecule is very rich in charged and has few large hydrophobic amino acids which causes it to bind SDS poorly. The same GAP43 preparation will also give a different SDS-PAGE molecular weight depending on the percentage acrylamide content of the gel, the protein appearing relatively larger on gels with higher acrylamide concentration. GAP43 proteins from different species also may run at different apparent molecular weights on the same gel. Partly due to these unusual features GAP43 was independently discovered by several different groups and therefore has several alternate names, such as protein F1, pp46, neuromodulin, neural phosphoprotein B-50 and calmodulin-binding protein P-57, in each case the number reflects the apparent SDS-PAGE molecular weight (2). GAP43 has been extensively studied and is a major protein kinase C substrate and binds calmodulin avidly, this being mediated by an N-terminal IQ calmodulin binding motif (3). GAP43 may also be anchored to the plasma membrane by reversible palmitoylation on two Cys residues close to the N-terminus (4). Knock out of the GAP43 gene in mice is lethal early in postnatal life and is associated with defects in axonal pathfinding (5). GAP43 is one of a large family of “intrinsically disordered proteins” which typically have little defined structure unless they are bound to a more structured partner (6).The MCA-5E8 antibody was made against the full length recombinant human protein and binds to GAP43 in rodents and other mammalian species. It binds strongly to growth cones and axonal processes of neurons in cell culture and to synaptic regions in sectioned material.We also supply chicken polyclonal and mouse monoclonal antibodies to GAP43, CPCA-GAP43, MCA-3H14 and MCA-5E8.