[1] Univ Penn, Anim Biol, Philadelphia, PA 19104 USA
[2] Univ Penn, Philadelphia, PA 19104 USA
[3] Univ Penn, Sch Vet Med, Dept Anim Biol, Philadelphia, PA 19104 USA
[4] Univ Penn, Dept Med, Perelman Sch Med, Philadelphia, PA 19104 USA
[5] NIH, Tumor & Metastasis Biol Sect, Ctr Canc Res, Bethesda, MD 20892 USA
[6] Univ Penn, Sch Vet Med, Dept Pathobiol 3, Philadelphia, PA 19104 USA