This family consists of several bacterial and phage virulence-related outer membrane proteins which are essential for the bacterial survival within macrophages and for eukaryotic cell invasion. These proteins consist of 8 transmembrane ß sheets and 4 cell surface-exposed mobile loops, as suggested by the structure of OmpX of Escherichia coli. The Ail protein of Yersinia enterocolitica is also a known virulence factor. It is thought that it directly promotes invasion and loop 2 contains an active site, perhaps a receptor-binding domain. The phage protein Lom is expressed during lysogeny and encodes host-cell envelope proteins. PagC of Salmonella typhimurium is required for survival in macrophages and for virulence in mice.