A RIBONUCLEIC acid fraction, which gives the characteristic sedimentation pattern shown in Fig. 1, has been isolated very recently from tobacco leaves1, mouse brain2 and microsomes of rat

liver1. The molecular weights of components A and B are 1.7 × 106 and 0.6 × 106, respectively. Although the study has yet to be extended to other systems, I suggested two years ago that

ribonucleic acids with molecular weights of about 1.7 × 106 exist in the cytoplasm of many types of cells3. The question that naturally arises is whether ribonucleic acids of such a

molecular weight also exist in a nucleus. So far no investigation along this line has been reported. The results presented below appear to provide an answer to the question.