Comment on "Evidence of Abundant and Purifying Selection in Humans for   Recently Acquired Regulatory Functions"
  Ward and Kellis (Reports, September 5 2012) identify regulatory regions in the human genome exhibiting lineage-specific constraint and estimate the extent of purifying selection. There is no statistical rationale for the examples they highlight, and their estimates of the fraction of the genome under constraint are biased by arbitrary designations of completely constrained regions. 
Nicolas Bray and Lior Pachter
q-bio.GN q-bio.QM