1 Laboratory of Metabolism, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA. Electronic address: kyunglee@mail.nih.gov.
2 Laboratory of Biomolecular Research, Division of Biology and Chemistry, Forschungsstrasse 111, CH-5232 Villigen PSI, Switzerland. Electronic address: michel.steinmetz@psi.ch.