Catalyzes the conversion of inosine 5'-phosphate (IMP) to xanthosine 5'-phosphate (XMP), the first committed and rate-limiting step in the de novo synthesis of guanine nucleotides, and therefore plays an important role in the regulation of cell growth (PubMed:7763314, PubMed:7903306). Could also have a single-stranded nucleic acid-binding activity and could play a role in RNA and/or DNA metabolism (PubMed:14766016). It may also have a role in the development of malignancy and the growth progression of some tumors. {ECO:0000269|PubMed:14766016, ECO:0000269|PubMed:7763314, ECO:0000269|PubMed:7903306}. This is the function of IMPDH2 (inosine monophosphate dehydrogenase 2, ENSG00000178035).