NK cells were treated with SNAP as described above. 20 μg of anti-nitrotyrosine beads (Millipore 16310) were added to 50 μg of protein lysate and incubated overnight at 4°C. The beads were washed with cold PBS and re-suspended in 2X Laemelli buffer and boiled. The sample was loaded on a SDS gel for protein expression by immunoblot analysis. The membrane was probed with an anti-Erk antibody (Cell Signaling) (28 (link)).