Picro-Sirius Red kit is used for visualization and collagen staining of tissues fixed in formalin and embedded in paraffin. It contains Sirius Red dye used in histology for demonstrating collagen fibers. Strong linear anionic dye stains collagen at low pH by reacting with base groups on collagen molecules. Sirius Red parallelly binds to collagen molecule; it also enhances collagen birefringence so it can be easily identified by observing the section under polarized light. Kit contains Eriochrome cyanine R dye that counterstains nuclei grey-brown. Eriochrome is acid-fast; that prevents nuclei from being discolored after longer incubation in Picro-Sirius Red solution.