ABSTRACT
NOR staining uses silver stain, which stains active nucleolar (or nucleolus) organizer regions on acrocentric chromosomes. The nucleolar organizer regions of mammalian chromosomes are known to contain the genes for 1 8S and 28S rRNA. Regions in which the genes are thought to be actively tran scribed can be selectively stained using silver nitrate (3 1 -33) . The stain is thought to selectively identify a protein adjacent to the nucleolar organizer region rather than the nucleolar organizer regions themselves . For human chromosomes, the genes for 1 8S and 28S rRNA are located on the short arms of the acrocentric chromosomes, which are chromosomes 1 3 , 1 4, 1 5 , 2 1 , and 22. With silver staining, NOR bands appear a s one or more dotlike structures of varying size located on the stalks (not the satellites) of the acrocentric chromosomes. Heritable polymorphic variations are often ob served using NOR staining with silver nitrate. Many feel that silver staining plays a crucial role in discriminating small bisatellited marker chromosomes
1 33
advantage of allowing an additional banding technique to be carried out on the same slide after the silver staining has been completed.
