Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a major threat to barley and caused devastating reductions in yield and economic losses. The barley line Q21861 was identified as the best source of resistance to the potential threat posed by the highly virulent Pgt race TTKSK. Resistance to TTKSK was mapped to the telomeric region of chromosome 5HL, now designated as the rpg4-mediated resistance locus (RMRL). RMRL has been delimited into two tightly linked yet distinct loci, designated as RMRL1 and RMRL2. Three genes (HvRga1, Rpg5, and HvAdf3) were found at RMRL1 while gene(s) at RMRL2 remains unknown. BSMV-VIGS revealed that all three genes at RMRL1 are required for rpg4-mediated resistance, which follows the emerging theme of dual genetically linked NBS-LRR genes required for resistance. HvAdf3 may play an important role in controlling host-specific resistance as suggested by AtADF4 in the Arabidopsis-Pseudomonas syringae plant-microbe interaction model.