Olivier Pourquié1, Katherine Brown2. 1. Harvard Medical School, Department of Genetics, Brigham and Women's Hospital, Department of Pathology, HIM118, 4 Blackfan Circle, Boston, MA 02115, USA pourquie@genetics.med.harvard.edu katherine.brown@biologists.com. 2. The Company of Biologists, Bidder Building, Station Road, Histon, Cambridge CB24 9LF, UK pourquie@genetics.med.harvard.edu katherine.brown@biologists.com.