Sinefungin, a polar nucleoside antifungal active with broad biological activity, was isolated from a number of species of Streptomyces in the early 1970s. Sinefungin acts by inhibiting a wide range of methyltransferases, including the methylation of bases in DNA and RNA which alters cytosine deamination and gene expression. Sinefungin is widely used as a bioprobe to block methyltransferase-dependent pathways.