The rice disease resistance gene Xa21 encodes a receptor kinase-like protein that plays a key role in plant defense against pathogens. The Xa21 gene was isolated through positional cloning and shown to confer resistance to Xanthomonas oryzae pv. oryzae race 6. The protein product of Xa21 contains a leucine-rich repeat (LRR) motif and a serine-threonine kinase-like domain, suggesting it is involved in cell surface recognition of pathogen ligands and activation of intracellular defense responses. The study demonstrates that the plant receptor kinase-like protein XA21 has a specific function in pathogen recognition and response. The Xa21 gene is located on chromosome 11 and is the only known locus conferring resistance to Xoo race 6. The gene was cloned and characterized, revealing a large open reading frame interrupted by an intron. The deduced amino acid sequence of XA21 shows similarity to known protein domains, including a signal peptide, LRR, transmembrane region, and a serine-threonine kinase domain. The protein is likely to function in disease resistance by mediating cellular signaling processes. The study also highlights the importance of understanding plant disease resistance mechanisms for developing engineered resistance in rice. The results suggest that the Xa21 gene functions in diverse rice species and that genes encoding subsequent signal transduction components are conserved. The findings have implications for future engineering and transfer of the Xa21 resistance gene to reduce losses from Xanthomonas infection in recipient species.The rice disease resistance gene Xa21 encodes a receptor kinase-like protein that plays a key role in plant defense against pathogens. The Xa21 gene was isolated through positional cloning and shown to confer resistance to Xanthomonas oryzae pv. oryzae race 6. The protein product of Xa21 contains a leucine-rich repeat (LRR) motif and a serine-threonine kinase-like domain, suggesting it is involved in cell surface recognition of pathogen ligands and activation of intracellular defense responses. The study demonstrates that the plant receptor kinase-like protein XA21 has a specific function in pathogen recognition and response. The Xa21 gene is located on chromosome 11 and is the only known locus conferring resistance to Xoo race 6. The gene was cloned and characterized, revealing a large open reading frame interrupted by an intron. The deduced amino acid sequence of XA21 shows similarity to known protein domains, including a signal peptide, LRR, transmembrane region, and a serine-threonine kinase domain. The protein is likely to function in disease resistance by mediating cellular signaling processes. The study also highlights the importance of understanding plant disease resistance mechanisms for developing engineered resistance in rice. The results suggest that the Xa21 gene functions in diverse rice species and that genes encoding subsequent signal transduction components are conserved. The findings have implications for future engineering and transfer of the Xa21 resistance gene to reduce losses from Xanthomonas infection in recipient species.