Nucleolar constitutive regions (NORs) are a segment of a chromosome that encodes ribosomal RNA (rRNA) and are found on DNA-specific loops that project into the nucleolus. Silver nitrate staining identifies the nucleolar constituent regions on tissue sections and reveals acidic proteins associated with NORs; however, these silver-stained NOR-associated protein (AgNOR) sites represent only part of the nucleolar constituent region in each nucleolus, not all of it, and electron microscopically the nucleolar constituent regions are poorly delineated, lightly stained areas in electron-dense zones. On paraffin sections, each punctate reactive granule seen in the nucleolus may represent more than one AgNOR site because normal or amphoteric nucleolus AgNORs tend to be tightly clustered, so that a single punctate granule after silver staining is in fact a cluster of multiple AgNORs.

The main features of silver protein staining solution for nucleolus constituent area are easy operation, more economical batch staining, and the increase in the number of AgNOR sites is related to the increase in cell proliferation, which is of some significance for the identification of benign and malignant tumors.