A new mechanism of clade B serpin gene expansion. (A) A new model for evolution of two clade B serpin loci in mammals, from a single locus of 4 genes. A series of intrachromosomal duplications results in 13 genes, followed by chromosomal breakage to yield two loci. Two previously proposed mechanisms based on duplication of an ancestral locus as described by (B) Bartuski et al. ; and (C) Scott et al. . Arrows represent gene duplication events. Black boxes indicate genes displaying the 8-exon structure, white boxes are 7-exon genes. Genes are named without the SERPIN root. Diagram not to scale.