The locus PKHD1 (polycystic kidney and hepatic disease 1) has been linked to all typical forms of the autosomal recessive polycystic kidney disease (ARPKD) and maps to chromosome 6p21.1-p12. We previously defined its genetic interval by the flanking markers D6S1714 and D6S1024. In our current work, we have fine-mapped the gene for the human P1 protein (MCM3), thought to be involved in the DNA replication process, to this critical region. We have also established its genomic structure. Mutation analyses using SSCP were performed in ARPKD patients' cDNA samples, leading to the exclusion of this gene as a candidate for this disorder. We also identified two intragenic polymorphisms that allowed families with critical recombination events to be evaluated. Although neither marker was informative in these individuals, they are the closest yet described for PKHD1 and may help to refine the candidate region.