[1] Harvard Univ, Sch Med, Ctr Blood Res, Boston, MA 02115 USA
[2] MIT, Ctr Canc Res, Cambridge, MA 02139 USA
[3] MIT, Dept Biol, Cambridge, MA USA
[4] MIT, McGovern Inst Brain Res, Cambridge, MA 02139 USA
[5] Harvard Univ, Sch Med, Dept Pediat, Boston, MA 02115 USA
[6] Univ Penn, Sch Med, Philadelphia, PA 19104 USA