ynfA, of Escherichia coli (P76169 swissprot, g1787865 GenBank) is the newest member of the Small Multidrug Resistance (SMR) gene family. It was identified with the aid of bioinformatics tools that allowed us to follow evolutionary principles of sequence conservation, in which functional similarity is inferred from sequence similarity, but surprisingly biochemical data yielded an unexpected mode of action for YnfA. The SMR gene family represents a prokaryotic transport system that is restricted to the eubacterial kingdom and has been so far identified in 63 bacterial species in both gram-negative and -positive bacterial species. Members of this family are composed of 100-120 amino acid residues in length and span the membrane four times as ?-helices. A ynfA inframe deletion mutant was created from an isogenic wild type E. coli MG1655 that displayed no growth defect compared to its wild type isogenic pair. Thus the mutant displayed antibiotic resistance.