This family is comprised of systems with unknown function found in prokaryotes. A typical system is made of 1 ABC, 1 IM and 2 proteins with a putative signal sequence (SS). The ABC is related to proteins of the MOI family but no significant similarity could be detected between SS and BP of BPD transporters. In Pseudomonas putida a Tn5 insertion within the ttg2A gene, encoding an MKL family ABC, renders the cells sensitive to toluene ( Pubmed : 10361295 ) . In Shigella flexneri, a mutation in the gene encoding the SS protein VspC was found to inhibit intracellular spreading of bacteria but not invasion, and to determine an increase in secreted virulence proteins and in sensitivity to sodium docecyl sulfate ( Pubmed : 9746567 ) . In M. tuberculosis, IM Rv0169 (Mcep) is a protein associated with the entry and survival of the bacterium inside cells ( Pubmed : 8367727 ) . MKL systems might be implicated in the maintenance of outer cell surface integrity.