Effect of PARP-1 knockout on the susceptibility of mouse 3T3 fibroblasts to the lethal effect of NCS. A (top). Cytotoxicity of NCS against PARP-1+/+ and PARP-1-/- 3T3s. The growth assays were performed as described under Methods. For PARP-1-/- 3T3s, the data were fitted to an exponential equation, S = e-α [NCS] where S is the surviving fraction (α = 0.398 ± 0.019 nM-1). For PARP-1+/+ 3T3s, a linear-quadratic equation was used in order to smooth the curve and gave the same value of α. The insert shows the response of PARP-1+/+ and PARP-1-/- 3T3s in the low range of NCS concentration. The amount of NCS (LC37) required to reduce cell survival to 1/e of that in control was 1.79 and 2.48 nM for PARP-1+/+ and PARP-1-/-3T3, respectively. B (bottom). Comparison of the response of PARP-1-/- 3T3s from a unique isolate transduced with a pBabe-puro retroviral vector carrying the full-length PARP-1 cDNA (pBabePARP-1) or the void vector (pBabevoid). Both cell lines showed the same exponentially-dependent response to NCS (α = 0.566 ± 0.044 nM-1).