[1] Stanford Univ, Sch Med, Dept Radiat Oncol, Stanford, CA 94305 USA
[2] Univ Penn, Abramson Family Canc Res Inst, Philadelphia, PA 19104 USA
[3] Univ Penn, Howard Hughes Med Inst, Philadelphia, PA 19104 USA
[4] Univ Calif San Diego, Div Biol Sci, La Jolla, CA 92093 USA