In tuberculosis-causing mycobacteria, the ESX-1 secretion system is known to be a key virulence determinant, with its absence causing severe attenuation, as seen in the ESX-1-deleted vaccine strains Mycobacterium bovis bacillus Calmette-Guérin (BCG) (23, 24) and Mycobacterium microti MP Prague (25). Here, ESX1 is linked to tuberculosis.