The esperamicins are a sub-family of enediynes that are considered among the most potent antitumor antibiotics discovered. First isolated in Actinomadura verrucosospora, members of the esperamicin family include esperamicin A1, A1b, A2, A3, A4, B1, B2, and X. Esperamicin X is an inactive esperamicin naturally produced by A. verrucosospora. Compounds with thiol groups induce triggering among the esperamicins.