A secreted protein that might play a role in virulence. Might serve as a chaperone to prevent uncontrolled membrane lysis by its partner EsxA. Forms a tight 1:1 complex with EsxA (PubMed:20085764). Probably secreted via the ESX-1 / type VII secretion system (T7SS). Belongs to the WXG100 family. CFP-10 subfamily.