Selective Pressure on the coding regions of bovine Type I IFN. Pairwise analysis of the coding region between every gene pair in a Type I IFN subfamily was used to calculate dS and dN, and the two values were plotted against one another. No comparisons were made between genes belonging to different subfamilies, i.e. an IFNB gene and an IFNA gene. In addition, all probable pseudogenes, which contained early stop codons or frameshift mutations, were not included in the analysis. The grey diagonal line in the graph represent neutral selection rate where dS = dN. Gene pairs undergoing positive selection would appear above the diagonal and gene pairs undergoing purifying selection would appear below the diagonal. The graph clearly shows that no positive selection is occurring within the coding regions of any Type I IFN family. Furthermore, IFNW and IFNA subfamilies appear to be undergoing purifying selection.