[1] Whitehead Inst Biomed Res, Cambridge, MA 02142 USA
[2] MIT, Dept Biol, Cambridge, MA 02139 USA
[3] Univ Massachusetts, Sch Med, Dept Biochem & Mol Pharmacol, Worcester, MA 01605 USA
[4] Univ Massachusetts, Sch Med, Program Gene Funct & Express, Worcester, MA 01605 USA
[5] Univ Colorado, Dept Chem & Biochem, Boulder, CO 80309 USA