The authors declare no competing interests exist.
Peter A. Combs conceived and designed the experiments, analyzed the data, and wrote the paper.
Michael B. Eisen conceived and designed the experiments and wrote the paper.
PAC is supported by National Institutes of Health training grant #T32 HG 00047. MBE is a Howard Hughes Medical Institute Investigator. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.