experimenter. (The three would share the 1969 Nobel Prize in Physiology or Medicine, "for work on the replication mechanism and genetics of viruses".) The phage group was an informal network of biologists centered on Max Delbrück that carried out basic research mainly on bacteriophage T4 and made numerous seminal contributions to microbial genetics and the origins of molecular biology in the mid-20th century. In 1961, Sydney Brenner, an early member of the phage group, collaborated with Francis Crick, Leslie Barnett and Richard Watts-Tobin at the Cavendish Laboratory in Cambridge to perform genetic experiments that demonstrated the basic nature of the genetic code for proteins. These experiments, carried out with mutants of the rIIB gene of phage T4, showed, that for a gene that encodes a protein, three sequential bases of the gene's DNA specify each successive amino acid of the protein. Thus the genetic code is a triplet code, where each triplet (called a codon) specifies a particular amino acid. They also obtained evidence that the codons do not overlap with each other in the DNA sequence encoding a protein, and that such a sequence is read from a fixed starting point. During 1962-1964 phage T4 researchers provided an opportunity to study the function of virtually all of the genes that are essential for growth of the phage under laboratory conditions. These studies were facilitated by the discovery of two classes of conditional lethal mutants. One class of such mutants is known as amber mutants. Another class of conditional lethal mutants is referred to as temperature-sensitive mutants Studies of these two classes of mutants led to considerable insight into numerous fundamental biologic problems. Thus understanding was gained on the functions and interactions of the proteins employed in the machinery of DNA replication, repair and recombination, and on how viruses are assembled from protein and nucleic acid components (molecular morphogenesis). Furthermore, the role of chain terminating codons was elucidated. One noteworthy study used amber mutants defective in the gene encoding the major head protein of phage T4. This experiment provided strong evidence for the widely held, but prior to 1964 still unproven, "sequence hypothesis" that the amino acid sequence of a protein is specified by the nucleotide sequence of the gene determining the protein. Thus, this study demonstrated the co-linearity of the gene with its encoded protein. A number of Nobel Prize winners worked with virus T4 or T4-like viruses including