1. 1Royal (Dick) School of Veterinary Studies and The Roslin Institute, University of Edinburgh, Easter Bush Campus, Midlothian, EH25 9RG, UK
2. 2Istituto Zooprofilattico Sperimentale delle Venezie, via dell’Universita` 10, 35020 Legnaro, PD, Italy
3. 3The Wellcome Trust Sanger Institute, Wellcome Trust, Genome Campus, Hinxton, CB10 1SA, UK
4. 4Department of Veterinary Medicine, University of Cambridge, Madingley Road, Cambridge CB3 0ES, UK