Possible origin of ABC1 porters from ABC2 porters. TMSs 3 and 4 of an ABC1 homologue, gi283948596 (top), aligned with TMSs 3 and 4 of an ABC2 homologue, gi149372921 (bottom), giving a comparison score of 11 S.D, 52.5% similarity and 39% identity. The numbers at the beginning of each line refer to the residue numbers in each of the proteins. TMSs are indicated in red lettering. Vertical lines indicate identities; colons indicate close similarities, and periods indicate more distant similarities.