The major virulence determinant in M. ulcerans is a polyketide-derived macrolide: mycolactone. Mycolactone was originally isolated from M. ulcerans 1615, a Malaysian isolate, as a mixture of cis/trans isomers designated mycolactone A and mycolactone B. Identical molecules were also found to be present in two M. ulcerans isolates from the Democratic Republic of Congo.[12] More recent evidence shows that M. ulcerans 1615 produces a family of mycolactone congeners which differ primarily in the number of hydroxyl groups and double bonds.[13]