The objective of the current study was to identify QTL conferring resistance toMarek's disease (MD) in commercial layer chickens. To generate the resourcepopulation, 2 partially inbred lines that differed in MD-caused mortality wereintermated to produce 5 backcross families. Vaccinated chicks were challengedwith very virulent plus (vv+) MD virus strain 648A at 6 d and monitored for MDsymptoms. A recent field isolate of the MD virus was used because the lines wereresistant to commonly used older laboratory strains. Selective genotyping wasemployed using 81 microsatellites selected based on prior results with selectiveDNA pooling. Linear regression and Cox proportional hazard models were used todetect associations between marker genotypes and survival. Significancethresholds were validated by simulation. Seven and 6 markers were significantbased on proportion of false positive and false discovery rate thresholds lessthan 0.2, respectively. Seventeen markers were associated with MD survivalconsidering a comparison-wise error rate of 0.10, which is about twice thenumber expected by chance, indicating that at least some of the associationsrepresent true effects. Thus, the present study shows that loci affecting MDresistance can be mapped in commercial layer lines. More comprehensive studiesare under way to confirm and extend these results.