A cell-permeable derivative of BB-Cl-amidine that contains an alkyne moiety for use in click chemistry reactions; inhibits PAD1-4 (Kinact/KI values of 6,400, 3,600, 10,800, and 4,900 M-1min-1, respectively); has been used for labeling PADs in cell-free and cell-based assays, followed by click reactions with azide-modified TAMRA or biotin reporters