The evolution of the snake-venom serine proteinase (SVSP) gene family in Crotalus adamanteus and C. horridus was characterized by duplication and changes in expression patterns. The bars adjacent to gene names give expression levels relative to the most highly expressed family member by species. A completely colored bar indicates the highest expression for the species. Gene-family expansion and contraction appear to be less pronounced in SVSPs than phospholipases A2 (Figure 3). Duplication and gene-loss events were inferred by means of a duplication-loss parsimony model. We used a homologous SVSP from the king cobra (Ophiophagus hannah) as an outgroup to root the phylogeny. Bayesian posterior probabilities are shown for clades for which the values exceeded 50%.