On 2013 Oct 26, Michael Eisen commented:

Would like to point to a followup paper from our lab: Lusk RW, 2010 used simulations of enhancer evolution to examine one of the conclusions of this paper - that the enrichment and conservation of paired binding sites we observe is the result of selection for paired sites - and found that a selection on binding site composition alone in the presence of a bias for deletions over insertions can result in an enrichment of clustered binding sites that appear (but are not) to be under purifying selection.

On 2013 Oct 26, Michael Eisen commented:

Would like to point to a followup paper from our lab: Lusk RW, 2010 used simulations of enhancer evolution to examine one of the conclusions of this paper - that the enrichment and conservation of paired binding sites we observe is the result of selection for paired sites - and found that a selection on binding site composition alone in the presence of a bias for deletions over insertions can result in an enrichment of clustered binding sites that appear (but are not) to be under purifying selection.