The molecular cloning of the first pathogen avirulence gene occurred several years later in the early 1980s while I was a research scientist at one of the first plant biotechnology companies, International Plant Research Institute (IPRI) (15). A seminar by Fred Ausubel on the construction of a Rhizobium cosmid library and the cloning of Rhizobium genes (nod genes) by genetic complementation inspired the development of a strategy to clone a bacterial avirulence gene. We hypothesized that we could clone a bacterial avirulence gene based on the fact that in previous genetic studies with fungal pathogens, Flor had shown that avirulence genes were genetically dominant. In collaboration with Noel Keen we set out to clone an avirulence gene by constructing the genomic library of a race 6 strain of Pseudomonas syringae pv.glycinea (Psg) in a wide host range cosmid cloning vector, pLARF1. We reasoned that we could test the idea whether avirulence genes could be detected in bacteria by conjugating a cosmid genomic library of Psg race 6 into a Psg race 5 strain. Because the two races have the reciprocal phenotype on two different cultivars of soybean, we could test whether virulence or avirulence was dominant by inoculating the exconjugants on both cultivars and scoring the phenotype