Domain organization of SelP and structures of SelP genes. (a) Domain organization of SelP sequences. Intron-exon structure of SelP is shown, and four domains are indicated. (b) Domain organization and homologous proteins. Fish (zebrafish is used as an example) and early mammals (for example, platypus) have two SelPs, a full-length protein SelPa and a shorter SelPb. Other mammals have only SelPa. Insects have either a short homolog that corresponds to exon 4 in mammalian SelP genes (for example, mosquito), or no homologs at all (fly). SP, signal peptide.