This work was supported by grant AGL2009-07003 from the Spanish Ministry of Sciences and Education (MEC) of Spain. P.R., D.P.-P. R.N., and E.G. were the recipients of grants from the MEC. We thank M. Álvarez from the Department of Biochemistry for his help in the radiolabeled experiments. Particularly, we would like to express our gratitude to A. Obaya for continuous support and help in our research and to L. Fernández for her previous work in IVET.