1. Departamentos de Bioquı́mica y Biologı́a Molecular and
2. Morfologı́a y Biologı́a Celular, Instituto Universitario de Oncologı́a, Facultad de Medicina, Universidad de Oviedo, 33006 Oviedo, Spain; and
3. Department of Vascular Biology, The Scripps Research Institute, La Jolla, California 92037