The HBM sensor domain has been identified primarily in bacterial chemoreceptors but is also present on histidine kinases. Characteristic features of this domain are its size of approximately 250 amino acids and its location in the bacterial periplasm. The McpS chemoreceptor of Pseudomonas putida KT2440 was found to possess an HBM sensor domain and its 3D structure in complex with physiologically relevant ligands has been reported. This domain is composed of 2 long and 4 short helices that form two modules each composed of a 4-helix bundle. The McpS chemoreceptor mediates chemotaxis towards a number of organic acids. Both modules of the McpS HBM domain contain a ligand binding site. Chemo-attractants binds to each of these sites and their binding was shown to trigger a chemotactic response. This domain is primarily found in different proteobacteria but also in archaea. Interestingly, amino acids in both ligand binding sites showed a high degree of conservation suggesting that members of this family sense similar ligands.