Intron-coded classification of serpin genes from vertebrates and lancelets and overview on intron gain positions. Vertebrate serpins are classified into six groups (V1–V6), based on group-specific sets of standard introns (black arrowheads). Characteristic representatives of each group are shown on the right. Non-canonical introns (marked in colors also used to indicate the genes concerned) are exclusively present in a lineage of ray-finned fishes, including Oryzias latipes (Japanese medaka), Gasterosteus aculeatus (stickleback), Tetraodon nigroviridis (green-spotted pufferfish) and Takifugu rubripes (Japanese pufferfish), but not in Petromyzon marinus and Lampetra fluviatilis (lampreys), Danio rerio (zebrafish), and tetrapods. Positions of introns (indicated on top) refer to human α1-antitrypsin, their phases (a-c) are given with respect to their location after the first, second or third base of the codon concerned. For comparison, serpins from lancelets (groups L1 to L3, intron positions indicated by grey arrowheads) have been included, demonstrating that there is little congruence concerning intron positions within the serpin superfamily. The intron at position 262c in group V5 (white arrowhead) is only found in fishes and was possibly lost in tetrapods. Some genes of group V1 lack the 85c intron. Some introns of L3 genes from B. floridae are restricted to individual members of this group (intron 280b: Spn9; introns 224b and 278b: Spn10). Due to alignment problems, the exact positions of the following introns are ambiguous: group V3, intron 86a-90a; group L1, intron 176a; group L2, intron 86b; group L3, intron 175c. Only introns mapping to the conserved serpin scaffold (amino acids 33 to 394 of human α1-antitrypsin) are considered.