Cyanobacteria, or blue-green algae, are a rich source of novel bioactive secondary metabolites that have potential applications as antimicrobial or anticancer agents or useful probes in cell biology studies. A Jamaican collection of the cyanobacterium Lyngbya majuscula has yielded several unique compounds including hectochlorin ( 1) and the jamaicamides A-C ( 5- 7). Hectochlorin has remarkable antifungal and cytotoxic properties. In this study, we have isolated the hectochlorin biosynthetic gene cluster ( hct) from L. majuscula to obtain details regarding its biosynthesis at the molecular genetic level. The genetic architecture and domain organization appear to be colinear with respect to its biosynthesis and consists of eight open reading frames (ORFs) spanning 38 kb. An unusual feature of the cluster is the presence of ketoreductase (KR) domains in two peptide synthetase modules, which are predicted to be involved in the formation of the two 2,3-dihydroxyisovaleric acid (DHIV) units. This biosynthetic motif has only recently been described in cereulide, valinomycin, and cryptophycin biosynthesis, and hence, this is only the second such report of an embedded ketoreductase in a cyanobacterial secondary metabolite gene cluster. Also present at the downstream end of the cluster are two cytochrome P450 monooxygenases, which are likely involved in the formation of the DHIV units. A putative halogenase, at the beginning of the gene cluster, is predicted to form 5,5-dichlorohexanoic acid.