Histonedeacetylases are promising molecular targets for the development of antitumor agents. A novel series of histonedeacetylaseinhibitors of the hydroxamic acid type were synthesized for structure-activity studies. Thirteen tricyclic dibenzo-diazepine, -oxazepine, and -thiazepine analogues were studied and shown to induce variable degrees of histone H3/H4 and tubulin acetylation in a cellular