The purine and pyrimidine base sequence of DNA , permanent repository of the genetic information, must be transcribed on to ribopolynucleotides before genotype can be translated into phenotype. This transcription gives rise to three recognizably different classes of RNA molecules: (1) two species of ribosomal RNA , one about 1500 and the other 3000 nucleotides in length, that form part of the structural members of the engine for cellular protein synthesis; (2) several dozen species of transfer RNA , each about 100 nucleotides in length, that provide adaptors in protein synthesis for the twenty ‘standard’ amino acids; and (3) hundreds, or thousands, of species of messenger RNA , probably of variable length, but reaching into the tens of thousands of nucleotides, that furnish the templates for orderly copolymerization of amino acids into specific polypeptides. To the particular purine and pyrimidine base sequence of every one of these multifarious RNA species there corresponds some homologous sector of the DNA where this sequence recurs in one of the complementary deoxypolynucleotide strands. It is generally assumed that the DNA strand of base sequence complementary to that of the RNA transcript acts as the template in transcription, because, in analogy with the mechanism of DNA replication, one imagines that formation of the complementary base pairs (Watson & Crick 1953) is responsible for specific alinement of the ribonucleotide monomers. But this point has not yet been firmly established, at least for the actual in vivo transcription. It would be prudent, therefore, to keep in mind for the time being that for transcription some other basepairing mechanism might conceivably obtain. For instance, pairing might also involve identical, rather than complementary, bases (Donohue & Stent 1956), in which case the DNA strand of identical base sequence could act as transcription template, or pairing might involve base triplets, rather than pairs (Stent 1958), in which case both DNA strands of the homologous sector could act jointly as transcription template.