– Protein arginine deiminases (PADs) catalyze the posttranslational modification of arginine residues on proteins to form citrulline, which plays a large role in regulating gene expression. Abnormally high PAD activity has been observed in a host of human diseases. Citrulline-specific probe-rhodamine is a highly sensitive, rhodamine phenylglyoxal-based fluorophore that specifically detects protein citrullination via a chemoselective reaction between glyoxal and citrulline. This chemical probe (comprised of a single isomer) is capable of reacting with any citrulline-containing protein and can be analyzed with fluorescent imaging (excitation 532 nm; emission 580 nm). When added at 100 μM for 30 minutes at acidic pH, this probe has a reported limit of detection of ~10 ng for citrullinated histone H3 and ~1 ng for autodeiminated PAD4.