Serine hydroxymethyltransferase (SHMT) is an enzyme (EC 2.1.2.1) which plays an important role in cellular one-carbon pathways by catalyzing the reversible simultaneous conversions of L-serine to glycine (retro-aldol cleavage) and tetrahydrofolate to 510-methylenetetrahydrofolate (hydrolysis). This reaction provides the largest part of the one-carbon units available to the cell.