Effect of neutral region choice in estimating the X-to-autosome effective population size ratio. Nx/Na (y-axis) is estimated by means of the X-to-autosome ratio of nucleotide diversity (π) normalized by human-macaque divergence to correct for variation in mutation rates. Bars are as described in text. Error bars are standard errors estimated by bootstrapping 10,000 data sets. YRI, Yoruba (Ibadan, Nigeria); CEU, CEPH (from Utah with Northern and Western European ancestry).