Early genetic studies in the 1980s carried out in the group of Nick Panopoulos resulted in the identification of bacterial genes controlling the ability of phytopathogenic bacteria to elicit a plant infection (9). These genes were termed “Hrp” genes, as bacteria containing these mutations concomitantly lost the ability to induce a HR on resistant plants and to cause disease in susceptible plants. Hrp genes were subsequently identified and characterized in several phytopathogenic genera, including Pseudomonas (Collmer group), Xanthomonas (Bonas group), Erwinia (Beer group), and Ralstonia (Boucher group). The Hrp genes are generally organized as 23- to 25-Kb clusters of DNA that contain several operons and have been subdivided into group I (Erwinia and Pseudomonas) and group II (Xanthomonas and Ralstonia) based on DNA homology (1, 8). Subsequent work revealed that some avirulence genes were actually within or near Hrp loci. It is interesting that it was also revealed that Hrp genes and avr genes in Pseudomonas syringae are coregulated. However, how Hrp genes and avr genes were functionally related remained a mystery.