Mysterin, also called RNF213, is a large intracellular protein (591 kDa) that contains two tandem AAA+ ATPase modules and a RING finger ubiquitin ligase motif. Although its physiological role remains unclear, mysterin is considered to be the most prominent susceptibility factor for moyamoya disease, an idiopathic cerebrovascular disorder in humans. The C-terminal R4810K mutation of mysterin significantly increases the risk of moyamoya disease, but its molecular effect on the mysterin protein remains largely unclear. Multiple studies have explored the physiological and pathological roles of mysterin at the molecular, cellular, and tissue/individual levels since its identification and molecular cloning, and the results obtained to date suggest that mysterin is involved in angiogenesis and/or endothelial cell behavior; however, no unified and precise understanding has yet been established. In this chapter, we provide an overview of mysterin’s genomic composition, enzymatic activities, structure, and mutant phenotypes in vitro and in vivo and discuss its potential physiological and pathological roles from the standpoint of molecular biology.