1. Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, NY 10065, USA.
2. Cell Biology Program, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, NY 10065, USA.
3. Department of Pharmacology and Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, NY 10065, USA.
4. Department of Chemistry, Columbia University, Havemeyer Hall, 3000 Broadway, New York, NY 10027, USA.