The highly oxygenated tetracyclic core skeleton of paclitaxel constitutes a major portion of the paclitaxel molecule. March this year revealed biosynthetic pathways in engineered yeast for some of the tri-oxygenated taxoids, discovering 9α-hydroxylase for a hydroxylation step. In addition, CYP725A55 was identified as catalyzing the oxetane ester formation via a cascade oxidation-concerted acyl rearrangement mechanism. Other discoveries included an acetyltransferase catalyzing the formation of C7-OAc from .