Establishing a successful interaction between legumes and rhizobia requires signal recognition between the two symbiotic partners. Thus, the evolution of symbiosis spe- cificity involves both rhizobial and host genes. From the bacterial side, specificity determinants include Nod factors, surface polysaccharides, and secreted proteins. However, we know relatively less from the host side. Per- ception of the Nod-factor signal is mediated by direct binding to the host Nod factor receptors (NFRs), which are plasma membrane-localized receptor kinases con- taining LysM motifs in their extracellular domains [49]. The role of NFRs in regulating host specificity was dem- onstrated by transferring the L. japonicus versions of NFRs to M. truncatula, which enabled the transgenic M. truncatula roots to nodulate with the L. japonicus symbiont Mesorhizobium loti [24]. However, natural variation in NFRs that causes changed specificity has rarely been documented. In contrast, numerous domin- ant genes have been identified in soybeans and other le- gumes that restrict nodulation with specific rhizobial strains [20,30]. We recently cloned the two soybean genes Rj2 and Rfg1 that restrict nodulation with specific strains of Bradyrhizobium japonicum and Sinorhizobium fredii, respectively [20]. We demonstrated that Rj2 and Rfg1 are allelic genes encoding a member of the Toll- interleukin receptor/nucleotide-binding site/leucine-rich repeat (TIR-NBS-LRR) class of plant resistance (R) pro- teins. Our discovery is consistent with recent reports that documented a large number of secreted effectors delivered into the host cell by rhizobial T3SS and sug- gests that establishment of a root nodule symbiosis re- quires the evasion of plant immune responses triggered by rhizobial effectors.