Forms a tight complex with EsxS (PubMed:15336430). Exists in heterodimeric and heterotetrameric forms (PubMed:20629176). Probably secreted via the ESX-3 / type VII secretion system (T7SS). In the heterotetrameric form, EsxS adopts a long helical conformation that pairs with a second EsxS monomer in an antiparallel manner that allows the N- and C-termini of two EsxS subunits to interact with an EsxR subunit at each end of the EsxS dimer thus creating a molecule with four-helix bundles at its extremities. Belongs to the WXG100 family. ESAT-6 subfamily.