Prosaposin (PSAP) is also named as Sulfated glycoprotein 1 (SGP-1). PSAP is a secreted neurotrophic factor and a lysosomal protein serving as the precursor of saposins (A-D), which are cofactors for hydrolases of sphingolipids (PMID: 25130661, PMID: 31220974). PSAP modulates glycosphingolipid metabolism and variants have been linked to Parkinson's disease (PD) (PMID: 37726325). As a dual-function protein, PSAP is a lysosome-associated protein that participates in a variety of cellular processes. In the lysosome, PSAP is processed to activate enzymes that degrade lipids. PSAP proteins located extracellularly are involved in cancer progression, such as proliferation and tumor death suppression signaling (PMID: 39405903). Prosaposin (PSAP) has two forms: a precursor and a secreted form. The secreted form has neurotrophic, myelinotrophic, and myotrophic properties. The precursor form is a precursor protein of saposins A-D (PMID: 33242172). Both human and mouse prosaposin are 65-70 kDa proteins cleaved in the lysosomes into four 10-15 kDa proteins designated saposins A, B, C and D (PMID: 8573994). Sulfated glycoprotein-1 (prosaposin) exists in 2 forms: a 65 kDa form targeted to lysosomes and a post-translationally modified to the 70 kDa form secreted extracellularly (PMID: 8593668).