Iakoubova, Olga, Holly Dushkin, Lisa Pacella, and David R. Beier. Genetic analysis of modifying loci on mouse chromosome 1 that affect disease severity in a model of recessive PKD. Physiol. Genomics: 101–105, 1999.—Using a cross between C57BL/6J and FVB/N mice, we have confirmed the localization on chromosome 1 of a modifying locus that affects the severity of polycystic kidney disease (PKD) in the juvenile cystic kidney (jck) mouse. Despite the highly significant contribution of this locus in F2 progeny of a cross between C57BL/6J and DBA/2J mice , a series of congenic strains carrying regions of chromosome 1 on a DBA/2J background did not show a severe disease phenotype. One possible explanation for these results is that this phenotype is caused by two linked loci, which have been separated in the congenic lines that were generated. This hypothesis is supported by the demonstration that severe PKD occurs in mice carrying a large congenic interval.