[1] Univ Alabama Birmingham, Dept Biochem & Mol Genet, Sch Med, Birmingham, AL 35294 USA
[2] Univ Alabama Birmingham, Dept Biochem & Mol Genet, Sch Dent, Birmingham, AL 35294 USA
[3] MIT, Dept Biol, Cambridge, MA 02142 USA
[4] Whitehead Inst Biomed Res, Cambridge, MA 02142 USA