1. Department of Cell and Molecular Neurobiology, Instituto de Investigaciones Biologicas Clemente Estable, Montevideo, Uruguay.
2. Department of Cell and Molecular Biology, Facultad de Ciencias, Montevideo, Uruguay.
3. Present address of Dr. Sotelo-Silveira: Laboratory of Molecular Technology, ATP, SAIC-National Cancer Institute, Frederick, Maryland.
4. Environmental Health Sciences Center, Oregon State University, Corvallis, Oregon.
5. Institut Pasteur de Montevideo, Montevideo, Uruguay.