Inosine-5'-monophosphate dehydrogenase 1, also known as IMPDH1 catalyzes the synthesis of xanthine monophosphate (XMP) from inosine-5'-monophosphate (IMP). Also, IMPDH1 is a ubiquitously expressed enzyme, functioning as a homotetramer, which catalyzed the rate-limiting step in de novo synthesis of guanine nucleotides. As such, it plays an important role in cyclic nucleoside metabolism within photoreceptors.