The snPI RNA species are a recently described set of molecules whose sizes range from 5S to 10S. They can be labeled in vitro in isolated nuclei and are apparently formed by an RNA polymerase I type of activity. However, in contrast to ribosomal precursor RNA, the usual polymerase I product, they are not found in the nucleolus but rather are located in the nucleoplasm. The snPI RNAs have been found in all mammalian cell types studied. The spectrum seen in gel electrophoresis is unique to each animal species studied but is essentially the same in different cell types within a species. The differences in snPI patterns are quite large between even closely related species and are clearly distinguishable in gorilla and human cells.

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