Protein Arginine Deiminase 4 (PAD4) is a guanidino-modifying enzyme that functions as a transcriptional coregulator catalyzing the conversion of specific arginine residues to citrulline. Substrates for PAD4 include histones H2A, H3, and H4. PAD4 autocitrullinates itself at several sites, inhibiting its enzymatic activity. PAD4 activity is increased in rheumatoid arthritis, producing an abundance of citrulline-containing proteins that generate an immune response resulting in production of autoantibodies that ultimately attack the host tissues. PAD4 has also been implicated in several other diseases including multiple sclerosis, Alzheimer's disease, glaucoma, and cancer. Cayman's PAD4 Inhibitor Screening Assay (Ammonia) provides a convenient, fluorescence-based method for screening human PAD4 inhibitors.