Methyl Green-Pyronin-Y Stain Solution is intended for used in differentiating DNA and RNA in tissue sections for the diagnosis of general pathology specimens. The reaction is based upon the competition between the slow staining, but doubly charged, methyl green and the more rapidly staining, singly charged pyronin Y. Methyl green has two cationic charged groups that become linked to the phosphate moieties in the DNA. The pyronin Y displaces the methyl green from all sites of linkage except where its double charge gives it a selective advantage (acidic polymer such as DNA). Consequently, the methyl green stains DNA and retains its binding to this substance against the competitive action of pyronin Y. Pyronin Y stains the less polymerized RNA rapidly and displaces methyl green from linkages having smaller polymeric acidic substances (RNA). The reaction is done at pH 4.2 – pH 4.3. The advantage of a low pH is to have the nucleic acids in a charged condition and in their least soluble state.