There are also many categories of unknown function that are greatly expanded in the Mtb clade relative to the nonpathogenic Mycobacteria (Tables 2 and 3 , red). For example, Rv0918 (in the Pfam group of unknown function PF08681) was found in a genetic screen that facilitates isolation of mutants defective in arresting the maturation of phagosomes [55] , helping Mtb to survive within host cells. PF07161 contains four lipoproteins (LprF, LprG, LprA, LppX). LprG and LppX were found to be in vivo essential genes by TraSH analysis [56] .