1. Department of Cell Biology and Physiology, Washington University, School of Medicine, St. Louis, MO 63110.
2. Instituto de Histologia y Embriologia, Facultad de Ciencias Medicas, Universidad Nacional de Cuyo (CONICET), Mendoza 5500, Argentina.
3. Section of Cell Growth, Regulation and Oncogenesis and Department of Biochemistry, Duke University Medical Center, Durham, NC 27707, USA.