horseradish peroxidase

noun

horse·​rad·​ish peroxidase -ˌrad-ish-, -ˌred- 

: a peroxidase that occurs in the root of the horseradish (Armoracia lapathifolia of the mustard family), that produces a distinctive color when exposed to an appropriate solution, and that is used as a label for antibody-antigen complexes, as a cytochemical marker in immunohistochemical staining, and as a tracer to follow the course of individual neurons since it is transported by most neural pathways