Myrosinase (0-thioglucosidase, thioglucoside glucohydrolase, EC 3.2.3.1) catalyzes the hydrolysis of glucosinolates, a class of sulfur-containing glycosides present in all Crucifers. Although intact glucosinolates are relatively nontoxic, their breakdown products (isothiocyanates, nitriles, or thiocyanates) have important biological influences on mammals, insects, and microbial pathogens (for review, see Fenwick et al., 1983). Myrosinase has been purified from a number of Crucifer species and found to be a glycoprotein of 135,000 mol wt consisting of two subunits of 65,000 mol wt (Bjorkman, 1976; Bones and Slupphaug, 1989). In intact plant tissues, myrosinase is sequestered in specialized cells called myrosin cells. When plant tissues are damaged as the result of pathogen or herbivore attack, myrosinase and glucosinolates come into contact, causing hydrolysis of glucosinolates to the biologically active compounds. Myrosinase cDNA sequences have been reported for Sinapis alba and Brassica napus (Falk et al., 1992; Xue et al., 1992). Southern blot analysis of genomic DNA isolated from S. alba (Xue et al., 1992), B. napus (Falk et al., 1992), and Brassica rapa (S. Machlin and D. Bradley, unpublished results) has shown that in these species myrosinase is encoded by large multigene families consisting of 6 to 14 genes. However,