1. Section of Postgraduate Studies and Research, Higher School of Medicine, National Polytechnic Institute, Mexico City 11340, Mexico
2. Departments of Cellular Biology, National Institute of Perinatology, Mexico City 11000, Mexico
3. Departments of Inmuno-Biochemistry, National Institute of Perinatology, Mexico City 11000, Mexico
4. Research Division, La Raza Medical Center, Mexican Instiitute of Social Security, Mexico City 02990, Mexico