The version of the peroxidase enzyme isolated from the horseradish plant. Like all peroxidases, it converts harmful peroxide molecules (H2O2) into water molecules (H2O). It is used to label proteins and nucleic acids as an alternative to radio labelling. The molecule to be labeled is attached to the horseradish peroxidase (HRP) molecule, and the mixture is then exposed to a substrate that changes from clear to colored when it is oxidized by HRP: H2O2 + Substrate-H2 --HRP--> 2H2O + Substrateox