S-methyl-5'-thioadenosine phosphorylase (MTAP) is an enzyme in humans responsible for polyamine metabolism. It is encoded by the methylthioadenosine phosphorylase (MTAP) gene on chromosome 9. Multiple alternatively spliced transcript variants have been described for this gene, but their full-length natures remain unknown. This gene encodes an enzyme that plays a major role in polyamine metabolism and is important for the salvage of both adenine and methionine. It is responsible for the first step in this pathway, where it catalyzes the reversible phosphorylation of MTA to adenine and 5-methylthioribose-1-phosphate. This takes place after MTA is generated from S-adenosylmethionine.