three mutant strains that produce ansamitocin and geldanamycin is combined with chemical synthesis, thus leading to 27 new ansamitocin derivatives. Structure–activity studies show that the N of the carbinolamide moiety is not important for cytotoxic activity but the α orientation of the OH group at C9 is key [see structures; chemical synthesis (red), mutasynthesis (blue), biosynthesis (black)].