Abstract: Epothilones A and B I (Fig. 1) are naturally occurring 16-membered macrolides, which are produced by the myxobacterium Sorangium cellulosum. [1] Although these compounds do not share any obvious structural similarities with the prominent anticancer drug paclitaxel (Taxol), they exhibit a similar biological profile in vitro, including the ability to inhibit microtubule depolymerization and to induce apoptosis in human cancer cell lines with sub-nM IC50s. [2] [3] Epothilone B is a 3-20-fold more potent inhibitor of human cancer cell growth than paclitaxel and unlike paclitaxel is also effective against multidrug-resistant cell lines. Epothilone B has demonstrated potent in vivo antitumor activity [3] in and is currently undergoing Phase I clinical trials by Novartis.