An exported protein. Plays a role in DNA conjugation, in at least a donor strain (PubMed:15314236). Forms a tight 1:1 complex with EsxA (PubMed:19854905, PubMed:20085764, PubMed:23150662). An artificial EsxA-EsxB heterodimer interacts with EspA (PubMed:19854905). Probably secreted via the ESX-1 / type VII secretion system (T7SS). Accumulates in the cell and is secreted when grown in Saunton's medium, when grown in 7H9 medium protein only accumulates intracellularly (PubMed:15687187). Increases efficiency of DNA conjugation when disrupted in donor strain (PubMed:15314236). Loss of detectable protein for EsxA and EsxB (PubMed:15687187). To improve expression in E.coli the proteins were cloned as a single protein in the (non-native) order esxA-esxB with a cleavable thrombin tag (PubMed:19854905). DNA conjugation in M.smegmatis is unidirectional with distinct donor and recipient strains; mc(2)155 is a donor strain while MKD8 is a recipient strain. Mutations in a donor strain that alter DNA transfer do not always alter DNA transfer in a recipient strain. Belongs to the WXG100 family. CFP-10 subfamily. Truncated N-terminus.