Receptor for medium-chain free fatty acid (FFA) with carbon chain lengths of C9 to C14. Capric acid (C10:0), undecanoic acid (C11:0) and lauric acid (C12:0) are the most potent agonists. Not activated by short-chain and long-chain saturated and unsaturated FFAs. Activation by medium-chain free fatty acid is coupled to a pertussis toxin sensitive G(i/o) protein pathway. May have important roles in processes from fatty acid metabolism to regulation of the immune system (By similarity). Expressed predominantly in hematopoietic tissues. Expressed mainly in the bone marrow with transcripts also detected in spleen, the lymph node, liver and the lung. By lipopolysaccharide in the monocyte/macrophage cell lines. Belongs to the G-protein coupled receptor 1 family.