Estimation of the intensity of natural selection acting on the various IFN families and their receptors. The strength of natural selection was assessed by estimating ω values. Under neutrality, ω is not significantly different from 1. Values below 1 are consistent with selection against nonsynonymous variants, whereas values greater than 1 indicate an excess of amino acid changes. Bars indicate 95% Bayesian confidence intervals, and red circles indicate genes with ω estimates significantly above or below 1. Genes shaded in gray correspond to those encoding the receptor subunits of each IFN family.