Stanford Genome Technology Center, Stanford University, Stanford, CA 94305, USA, Biological Sciences Division and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA and Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA Stanford Genome Technology Center, Stanford University, Stanford, CA 94305, USA, Biological Sciences Division and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA and Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.