Whitehead Institute, Center for Genome Research, 320 Charles Street, Cambridge MA 02141, USA. nick@genome.wi.mit.edu|Whitehead Institute, Center for Genome Research, 320 Charles Street, Cambridge MA 02141, USA. nick@genome.wi.mit.edu|Whitehead Institute, Center for Genome Research, 320 Charles Street, Cambridge MA 02141, USA. nick@genome.wi.mit.edu|Whitehead Institute, Center for Genome Research, 320 Charles Street, Cambridge MA 02141, USA. nick@genome.wi.mit.edu|Whitehead Institute, Center for Genome Research, 320 Charles Street, Cambridge MA 02141, USA. nick@genome.wi.mit.edu