Streptonigrin is a natural product produced by Streptomyces flocculus, has both anti-tumor and anti-bacterial activity. Streptonigrin acts as a pan-PAD inhibitor (IC50s: 48.3±34.2 μM, 26.1±0.3 μM, 0.43±0.03 μM, and 2.5±0.4 μM for PAD1, PAD2, PAD3, and PAD4, respectively).