This domain is found in the MbtH protein as well as at the N-terminus of the antibiotic synthesis protein NIKP1. MbtH and its homologs were first noted in gene clusters involved in non-ribosomal peptides and other secondary metabolites by Quadri et al. This domain is about 70 amino acids long and contains 3 fully conserved tryptophan residues. The structure of the PA2412 protein shows it adopts a beta-beta-beta-alpha-alpha topology with the short C-terminal helix forming the tip of an overall arrowhead shape. MbtH proteins have been shown to be required for the synthesis of antibiotics, siderophores and glycopeptidolipids.