IMPDH1 (inosine monophosphate dehydrogenase) is a ubiquitous cytosolic and nuclear enzyme that plays a central role in guanine nucleotide metabolism. It catalyzes the NAD-dependent conversion of inosine monophosphate (IMP) to hypoxanthine monophosphate (XMP) which is a precursor for the synthesis of GMP, guanosine, and guanine. These compounds are critical for DNA synthesis and cell proliferation. IMPDH1 is inhibited by the immunosuppressant drug mycophenolic acid (MPA) which induces reversible IMPDH1 aggregation. It is required for vascular endothelial cell proliferation and the terminal differentiation of adipocytes. Mutations of IMPDH1 are associated with autosomal dominant retinitis pigmentosa.