During the 1960s and 1970s, there was a major conundrum in eukaryotic molecular biology regarding the synthesis of mRNA in animal cells. Pulse and pulse-chase radio-labeling experiments with tritiated uridine had shown that a large fraction of nuclear RNA was degraded very rapidly after synthesis, whereas only a small fraction of the initially synthesized RNA was exported from the nucleus to the cytoplasm where it functions as far more stable mRNAs (1, 2). This short-lived nuclear RNA ranged in lengths up to several tens of kilobases, much longer than most cytoplasmic mRNAs, and was referred to as heterogeneous nuclear RNA (hnRNA) (3, 4). Pulse-chase labeling yielded results consistent with hnRNA functioning as a precursor of mRNA. hnRNA is rapidly labeled, whereas label appeared in mRNAs more slowly. Also, as for mRNA, a large fraction of hnRNA was shown to be polyadenylated at its 3′ end like mRNA, consistent with the model that RNA sequences near the 3′ end of hnRNAs are retained in shorter mRNAs exported to the cytoplasm (5, 6). Also, like mRNAs, hnRNAs were found to have the 7-methyl guanine 5′–5′ phosphotriester “cap” structure at their 5′ ends, like mRNAs. Remarkably, the methyl groups in hnRNA cap structures appeared to be conserved as RNA was exported to the cytoplasm, even though, on average, hnRNA molecules in the nucleus are at least four times longer than mRNAs in the cytoplasm (7, 8). However, it was not possible to prove rigorously from the flow of radiolabel in pulse-chase labeling experiments that hnRNAs are precursors to mRNAs. There were two principal reasons for this. First, the pool of ribonucleoside triphosphates in mammalian … [↵][1]1Email: berk{at}mbi.ucla.edu. [1]: #xref-corresp-1-1