HC-toxin is a cyclic tetrapeptide of structure cyclo(D-Pro-L-Ala-D-Ala-L-Aeo), where Aeo stands for 2-amino-9,10-epoxi-8-oxodecanoic acid. It is a determinant of specificity and virulence in the interaction between the producing fungus, Cochliobolus carbonum, and its host, maize. HC-toxin qualifies as one of the few microbial secondary metabolites whose ecological function in nature is understood. Reaction to C. carbonum and to HC-toxin is controlled in maize by the Hm1 and Hm2 loci. These loci encode HC-toxin reductase, which detoxifies HC-toxin by reducing the 8-carbonyl group of Aeo. HC-toxin is an inhibitor of histone deacetylases (HDACs) in many organisms, including plants, insects, and mammals, but why inhibition of HDACs during infection by C. carbonum leads to disease is not understood. The genes for HC-toxin biosynthesis (collectively known as the TOX2 locus) are loosely clustered over >500 kb in C. carbonum. All of the known TOX2 genes are present in multiple, functional copies and are absent from natural toxin non-producing isolates. The central enzyme in HC-toxin biosynthesis is a 570-kDa non-ribosomal synthetase encoded by a 15.7-kb open reading frame. Other genes known to be required for HC-toxin encode alpha and beta subunits of fatty acid synthase, which are presumed to contribute to the synthesis of Aeo; a pathway-specific transcription factor; an efflux carrier; a predicted branched-chain amino acid aminotransferase; and an alanine racemase.