Structurally related secondary products are rather rarely shared by organisms from different kingdoms. Consequently, the evolution of biosynthetic pathways of defence metabolites between distantly related organisms has not been broadly investigated. Thiazolylindoles are found in Arabidopsis thaliana, as the phytoalexin camalexin, and in a Streptomyces strain, which synthesizes a tumour-inhibitory derivative, designated BE-10988. Camalexin originates from cysteine and tryptophan, which is converted to indole-3-acetaldoxime and subsequently dehydrated to indole-3-acetonitrile. The metabolic origin of BE-10988 was determined by retrobiosynthetic NMR analysis and incorporation studies with direct precursors. Like camalexin, it is derived from tryptophan and cysteine. However, as BE-10988 is synthesized via indole-3-pyruvic acid, not via indole-3-acetaldoxime, independent mechanisms of tryptophan modification have evolved.