In some studies [29, 53–55], the attenuated phenotype of BCG-mycobacteria and their inability to replicate and/or leave phagosomal vacuoles in host cells in cell lines following in vitro infection were explained by the absence, in the BCG genome, of some genes that virulent strains of M. bovis and M. tuberculosis possess and, first of all, by the absence of the RD1 locus containing the genes coding for proteins of the secretion system ESX-1. Here, ESX1 is linked to infection.