Kifunensine is an immunoactive compound originally produced by Kitasatosporia kifunense, which displays competitive inhibition against immunosuppressive factor in tumor-bearing mice. Kifunensine has also been shown to be a potent inhibitor of the purified glycoprotein processing enzyme, mannosidase I (MAN1), specifically MAN1A1, MAN1A2, MAN1B1 and MAN1C1. Kifunensine inhibits human endoplasmic reticulum α-1,2-mannosidase I and Golgi Class I mannosidases IA, IB and IC with Ki values of 130 and 23 nM, respectively. Enzymes of this class are important factors for the biosynthesis of various types of N-linked oligosaccharide structures. Inhibition of these structures by kifunensine can interfere with cell-to-cell adhesion, targeting of lysosomal hydrolases to lysosomes, and clearance of asialoglycoproteins from the serum. Kifunensine is used to suppress Endoplasmic Reticulum-Associated Degradation (ERAD) via the inhibition of endoplasmic reticulum-associated mannosidase activity. Kifunensine has shown potential for treatment of sarcoglycanopathies and lysosomal storage disorders. Orphan designation (EU/3/11/906) was granted by the European Commission to Généthon, France, for kifunensine for the treatment of beta sarcoglycanopathy, but it was withdrawn later. Kifunensine’s use as a therapeutic is currently being researched in several conditions that benefit from its ability to inhibit mannosidase I.