Neutral rates, GC and distance to telomeres. (a-d) Scatter plots of human-macaque neutral substitution rates from non-CpG and all sites in ancestral repeats against human GC content ((a) and (b), respectively) and distance to telomeres ((c) and (d), respectively). Each point represents one of 2,270 windows of size 1 Mb. Lowess smoothers are superimposed to the plots to help visualize the relationships. These non-parametric fits reveal some curvature in the way GC content and distance to telomeres are related to neutral substitutions, which is consistent with the significant quadratic terms in our regression fits.