SHMT1 (serine hydroxymethyltransferase 1) is a cytosolic enzyme that plays a central role in one-carbon metabolism, catalyzing the reversible conversion of serine and tetrahydrofolate (THF) to glycine and 5,10-methylenetetrahydrofolate (5,10-methylene-THF). This reaction provides the primary source of one-carbon units for a variety of essential biosynthetic processes, including the synthesis of purines, thymidylate, and methionine, as well as for methylation reactions and redox balance maintenance. SHMT1 is highly expressed in proliferating tissues and cancer cells, where its activity supports increased nucleotide synthesis and cell growth. Additionally, SHMT1 is regulated at both transcriptional and post-translational levels, and its dysregulation has been linked to metabolic disorders, developmental defects, and cancer progression. The enzyme's critical role in compartmentalized one-carbon metabolism makes it an important target for therapeutic intervention, particularly in oncology.