Although only components of the transport machinery across the plasma membrane have been identified so far (12, –, 14), these secretion systems are thought to be specialized for the secretion of various protein substrates across the complex cell envelope (15, 16), with five functionally nonredundant ESX systems being present in Mycobacterium tuberculosis, the causative agent of tuberculosis, and only three being present in M. smegmatis (ESX-1, ESX-3, and ESX-4) (17). This evidence concerns the gene ELF3 and tuberculosis.