1. Department of Veterinary Medicine, Equine Fertility Unit, Mertoun Paddocks, University of Cambridge, Newmarket, Suffolk CB8 9BH, UK.
2. Present address: China Agricultural University, College of Animal Science and Technology, Equine Research Centre, No 2 Yangminyang Xilu, Beijing 100049, China.
3. The Babraham Institute, Babraham, Cambridge CB2 4AT, UK.
4. Clinical Research Centre, Huddinge University Hospital, Karolinska Institute, 141 57 Stockholm, Sweden.