A secreted protein. Acts as a strong host T-cell antigen. Plays a number of roles in modulating the host's immune response to infection as well as being responsible for bacterial escape into the host cytoplasm. Forms a tight 1:1 complex with EsxB (CFP-10). Probably secreted via the ESX-1 / type VII secretion system (T7SS). A central pair of alpha helices probably insert into host membrane. Genes esxA and esxB are part of RD1 (part of a 15-gene locus known as ESX-1), a section of DNA deleted in the M.bovis BCG strain used for vaccination. Deletion of this region is thought to be largely responsible for the attenuation of BCG, and esxA and esxB in particular are very important in this effect (PubMed:8631702). Belongs to the WXG100 family. ESAT-6 subfamily.