Published in: Proc. Natl. Acad. Sci. U.S.A., Vol. 52, No. 1, pp. 93-100 (July, 1964):

V. C. Littau, V. G. Allfrey, J. H. Frenster, and A. E. Mirsky.
The Rockefeller Institute
New York, NY 10021

Much of the DNA in the nucleus of a highly differentiated cell (the calf thymocyte) occurs in a condensed or compacted state, visible as electron-dense clumps in the electron microscope and as densely staining regions after Feulgen staining. A small part of the DNA occurs in a more diffuse, extended state.

When autoradiographs are prepared after labeling nuclear RNA with tritiated uridine, it is found that most of the radioactive RNA occurs in the diffuse chromatin areas, and that the condensed chromatin is virtually inactive in RNA synthesis. This is a direct demonstration of localized nuclear activity which verifies earlier conclusions that much of the DNA of the nucleus is repressed, as far as "messenger" RNA synthesis is concerned, and that the repressed DNA occurs in isolable clumps of condensed chromatin.