hanna, K. K. & Jackson, S. P. DNA double-strand breaks: signaling, repair and the 232 cancer connection. Nat. Genet. 27, 247-254 (2001). 233 22. Vilenchik, M. M. & Knudson, A. G. Endogenous DNA double-strand breaks: production, 234 fidelity of repair, and induction of cancer. Proc. NatL Acad. Sci. U.