2002; Bottai and Brosch 2009; Weerdenburg et al. 2015). In contrast to M. tuberculosis, M. marinum has almost two identical copies of the ESX-1 substrates esxAB. The extra set appears to be specifically necessary for the infection of the fish cell line CLC, suggesting a host-specific ESX-1 function (Weerdenburg et al. 2015). This evidence concerns the gene ESX1 and infection.