When growing cultures of mammalian cells are exposed to radioactive RNA precursors, two general types of very high molecular weight nuclear RNA can be recognised; ribosomal precursor RNA and a series of molecules, with molecular weight varying between 1 x 10⁶ and 20 x 10⁶, which have been collectively termed heterogeneous nuclear RNA (HnRNA). It is commonly believed that cytoplasmic messenger RNA is formed from nuclear HnRNA by post-transcriptional modification. The aim of this work was to examine the relationship between HnRNA and mRNA by studying short sequences present in them.