Silver staining of the nucleolus organizer regions (NOR) requires clusters of sulfhydryl groups.

Silver stainability of nucleolus organizer regions (NORs) appears to be correlated with the presence of grouped sulfhydryl (SH) side chains of proteins. In fact, heavy metals with high affinity for SH groups, such as Hg and Cu, do prevent the silver staining reaction. Ferricyanide, which is known to oxidize SH to disulfides, also prevents any further reaction with silver. On the other hand, alkali and reducing agents (mercaptoethanol, cyanide) do not affect silver stainability of the NORs. These results show that the silver staining reaction is not related to disulfide or persulfide groups and that alkali-soluble, acidic nuclear proteins per se do not play a major role in this process.