Mizoribine (MZB) is an imidazole nucleoside and an immunosuppressive agent. Eupenicillium brefeldianum, an ascomycetes harvested from the soil of Hachijo Island, Tokyo, Japan, in 1971, produces mizoribine (MZB). MZB is a nucleoside of the imidazole class, and was found to have weak antimicrobial activity against Candida albicans, but it proved ineffective against experimental candidiasis. MZB has been approved in Japan for the treatment of lupus nephritis (1990), rheumatoid arthritis (1992), and primary nephritic syndrome (1995), and in these diseases, it has often been used in combination with corticosteroids and/or anti-inflammatory drugs. Mizoribine also has been used in renal transplantation, and in steroid-resistant nephrotic syndrome. After phosphorylation, misoribine-5’-monophosphate (MZB-P) inhibits guanosine monophosphate (GMP) synthesis by antagonistic blocking of inosine monophasphate dehydrogenase (IMPDH) and GMP-synthetase in the pathway from inosine [ 5’-] monophasphate (IMP) to GMP in the purine synthesis system. MZB-P appears to almost completely inhibits guanine nucleotide synthesis Suppressive effect of MZB on cell growth is attributable to MZB-P and not to MZB itself. Thus, MZB has less toxicity than azathioprine, another immunosuppressant used for some of the same diseases.