Neurotrophins, like many growth factors, are expressed as precursor proteins which are then truncated and further modified to yield the mature form. The presence of highly conserved sequences in the prodomains of different neurotrophins made it clear that the prodomain must have some biological role and is not solely present as a protein folding chaperone. Proneurotrophins mediate their signaling by binding to p75^NTR and Sortilin receptors.