H-Gly-Arg-pNA hydrochloride (GR-pNA hydrochloride) is a colorimetric substrate for thrombin. H-Gly-Arg-pNA is preferentially bound by thrombin and cleaves the Gly-Arg (GR) peptide sequence, releasing p-nitroaniline (pNA) that can be detected colorimetrically at 405 nm. H-Gly-Arg-pNA hydrochloride is used as a fluorescent dye to measure thrombin activity.