1. Integrated Program in Cellular, Molecular, and Biophysical Studies, Columbia University College of Physicians and Surgeons, New York, NY 10032, USA,
2. Regeneron Pharmaceuticals, Inc., Tarrytown, NY 10591, USA.
3. Department of Genetics and Development, Columbia University College of Physicians and Surgeons, 701 West 168 Street, New York, NY 10032, USA.