The Broad Institute of MIT and Harvard, 415 Main Street, Cambridge, MA 02142, USA. melanie@broadinstitute.org.|University of Liverpool, Centre for Genomic Research, Crown Street, Liverpool, L69 7ZB, UK.|University of Glasgow, School of Engineering, Oakfield Avenue, Glasgow, G12 8LT, UK.|University of Liverpool, Centre for Genomic Research, Crown Street, Liverpool, L69 7ZB, UK.|University of Warwick, Warwick Medical School, Coventry, CV4 7AL, UK.