3. **Formation of the Taxol Tetracyclic Core Skeleton**: -The paper presents a groundbreaking discovery regarding the formation of the oxetane ester, an essential structural motif in paclitaxel. This is accomplished through the action of CYP725A55, which catalyzes a complex reaction mechanism involving oxidation and acyl rearrangement without the formation of an intermediate epoxide ester. -This mechanism is experimentally supported through isotope labeling experiments (using 18O), showing that the oxetane ring is formed directly via a concerted acyl rearrangement process, marking a significant advancement in understanding the formation of the core skeleton of paclitaxel. )-diene-5α-ol (2) using an engineered *Saccharomyces cerevisiae* strain. This represents a notable step towards fully reconstructing the Taxol biosynthetic pathway outside of natural plant sources, which is crucial for sustainable production. -Through the co-expression of 12 enzymes in the yeast strain, including T13OH, T5AT, TAX19, T10OH, CYP725A37, and CYP725A55, the biosynthesis of the Taxol tetracyclic core skeleton is achieved, laying a foundation for future work in engineering paclitaxel biosynthesis.