The YiaAB family (COG4682; 4 TMSs and COG4298; 2 TMSs) includes two paralogous 4 TMS proteins in E. coli which map adjacent to each other, YiaA (145 aas) and YiaB (117 aas). The topology of YiaB is established with both the N- and C-termini in the cytoplasm (Drew et al., 2002). This protein is apparently present only in E. coli and Shigella. In both YiaA and YiaB, peaks 1 and 3 are much more hydrophobic than peaks 2 and 4, and the first halves are homologous to the second halves. All homologues are from bacteria. No functional data are available for these proteins, and many organisms that possess homologues do not necessarily have two. Many bacteria have only YiaA, not a YiaB. Many bacteria have 2 TMS homologues. For example, Deinococcus radiodurans has a 95 aa homologue (AAF10670) which is predicted to have 2 TMSs, and it is the equivalent of the first half of YiaA. Another homologue in Herpetosiphon aurantiacus (94 aas; EAU15084) has 2 TMSs with a C-terminal hydrophilic extension. Another homologue from Xanthomonas oryzae (gi#58583127) is of 227 aas, has 2 TMS (COG4682) domains linked to a VgrG hydrophilic domain of unknown function. The VgrG domain is found in many bacterial proteins.