Synonymous distance between Smcx and Smcy in M. musculus. Nonoverlapping 100-nt windows were examined along the length of the aligned Smcx and Smcy exons. A dip in the Nei-Gojobori synonymous distance (p) occurs at ∼exons 23-24, marked with bar. Exon structure is mapped along the bottom of the graph, showing approximate exon-exon boundaries as vertical lines. Introns were not alignable between Smcx and Smcy; introns are thus not shown.