The enzyme from?Saccharomyces cerevisiae?is active only towards uridine38?and uridine39, and shows no activity with uridine40?(cf. EC?5.4.99.12, tRNA pseudouridine38-40?synthase) [1]. In vitro the enzyme from mouse is active on uridine39?and very slightly on uridine38?(human tRNALeu) [2].