1. Departamento de Bioquímica y Biología Molecular (A), Facultad de Veterinaria, Universidad de Murcia, Apdo. 4021, E-30100 Murcia, Spain, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden, and Division of Molecular Biometry, Institute for Cell and Molecular Biology, Uppsala University, SE-75 124 Uppsala, Sweden