The first identified member of this family was the salt-stress induced outer membrane protein SspA from Rhodobacter sphaeroides sp. denitrificans. It was found in the outer membrane acting as a transmembrane protein and was specifically induced by salt stress, but not by heat shock. Similar proteins were found later in many bacterial species from the genera Escherichia, Salmonella, Yersinia, Shewanella and Vibrio. Members of this family are predicted to possess a rather small barrel size compared to the general diffusion porins, while their biological role remains unknown.