Expression of WT PARP-1 provides some protection from cell death triggered by GzmB^−/− and GzmA^−/− CTLs, whereas mutation of the preferred PARP-1 GzmA cleavage site protects only against GzmB^−/− CTLs. Cell death of PARP-1–transduced ConA-coated PARP-1^−/− MEFs, attacked by GzmB-expressing CTLs (GzmA^−/−, left) or GzmA-expressing CTLs (GzmB^−/−, right), was measured by ⁵¹Cr release assay. (A) PARP-expressing cells are more resistant to CTL attack. K498A PARP-1–expressing cells are more resistant than WT PARP-1–expressing cells to attack by GzmA-expressing CTLs, but not by GzmB-expressing cells. (B) Expression of PARP^1-498 increases susceptibility to both GzmB^−/− and GzmA^−/− CTLs. Cells were transfected to overexpress WT (♦) or K498 (■) PARP-1 (A) or PARP-1^1-498 (▴) or PARP-1^499-1014 (•) (B) or with vector (×). Cell death of control cells, without CTLs, is indicated by the corresponding open symbols and dashed lines.