This family consists of homologs (found only in Borrelia species) of the p13 gene product of Borrelia burgdorferi, which is posttranslationally processed at both ends and modified by an unknown mechanism. This protein is a 13 kDa integral outer membrane protein and exhibits a channel-forming activity. This channel is cation-selective and voltage-independent. The exact structural features of these proteins are not fully elucidated, since conflicting predictions reported P13 to be alpha helical. Single channel and selectivity measurements demonstrated that P13 had no preference for either cations or anions and showed no voltage-gating up to ±100 mV. The localization to the outer membrane and the porin activity leads to the assumption that the protein must form a ß barrel.