Because of its fermentative ability, E. coli did not require ubiquinone for growth on glucose, making the isolation of ubiquinone mutants difficult. To overcome this problem, non-fermentable substrates such as succinate or malate were substituted for glucose and under these conditions cells unable to make ubiquinone failed to grow (32). Using nitrosoguanidine as a mutagen and a system of delayed enrichment on solid agar, ∼100 mutants were isolated that could grow on glucose but not on malate. After purification each one of these was grown in 2-L batches, the cells were extracted and the extracts run on chromatograms to detect the presence or absence of ubiquinone. Of the 100 mutants tested, two were found to be unable to synthesise ubiquinone (40, 44). The accompanying genetic analysis proved to be important when the first mutant strain to be isolated was shown to possess four separate mutations, each of which affected growth on malate and two of which involved lesions in the ubiquinone pathway. Genetic techniques were used to establish mutants with a single mutation affecting ubiquinone synthesis and these were then subjected to a detailed analysis. Genetic crosses with Hfr strains followed by transductions were used to locate the mutated genes on the E. coli chromosome and large-scale cultivation of the mutants produced sufficient quantities of the intermediate before the blocked reaction to allow a detailed spectral analysis with NMR, mass spectrometry and infrared spectroscopy. At this stage Cox and Gibson transferred their major interest to oxidative phosphorylation and ATP synthase and Ian Young took a major responsibility for the isolation and characterization of mutants blocked in the remaining five reactions and for the identification of the intermediates formed by these mutants.The overall results of these studies are summarized in Frank Gibson’s special lecture to a joint meeting of the Biochemical Society and the Chemical Society in London in 1972 (82), and in a paper by Young, Stroobant, McDonald and Gibson in 1973