The epothilones are a new class of non-taxane tubulin polymerization agents obtained by natural fermentation of the myxobacteria Sorangium cellulosum. The cytotoxic activities of the epothilones, like those of the taxanes, have been linked to stabilization of microtubules, but they also have important differences. Among the epothilone family, ixabepilone (BMS247550) is a semisynthetic derivative of the natural product epothilone B. Ixabepilone was evaluated in vivo in a panel of human and rodent tumour models, the majority of which were chosen because of their known, well-characterized resistance to paclitaxel, and seems able to overcome the over-expression of multidrug resistance and to be unaffected by mutations in the beta tubulin gene. The interest of ixabepilone was clinically confirmed in clinical studies of phase II which demonstrated a strong activity at the patients with metastatic breast cancer resistant to taxanes and in patients suffering of other types of chemoresistant tumors.