Mutations of PKD1 and PKD2 account for most cases of autosomal dominant polycystic kidney disease (ADPKD). Compared with PKD2, patients with PKD1 typically have more severe renal disease. Here, we report a follow-up study of a unique multigeneration family with bilineal ADPKD (NFL10) in which a PKD1 disease haplotype and a PKD2 (L736X) mutation co-segregated with 18 and 14 affected individuals, respectively. In our updated genotype–phenotype analysis of the family, we found that PKD1-affected individuals had uniformly mild renal disease similar to the PKD2-affected individuals. By sequencing all the exons and splice junctions of PKD1, we identified two missense mutations (Y528C and R1942H) from a PKD1-affected individual. Although both variants were predicted to be damaging to the mutant protein, only Y528C co-segregated with all of the PKD1-affected individuals in NFL10. Studies in MDCK cells stably expressing wild-type and mutant forms of PKD found that cell lines expressing the Y528C variant formed cysts in culture and displayed increased rates of growth and apoptosis. Thus, Y528C functions as a hypomorphic PKD1 allele. These findings have important implications for pathogenic mechanisms and molecular diagnostics of ADPKD.