Autosomal recessive polycystic kidney disease (ARPKD) is an often devastating form of polycystic kidney disease that presents primarily in infancy. The locus, PKHD1 (polycystic kidney and hepatic disease 1), on chromosome 6p21.1-p12, has been linked to all classical forms of this disorder. In previous studies, we cloned the PKHD1 interval in a set of overlapping YACs, converted this YAC-based framework into a BAC/PAC contig, and delimited the critical interval to a region flanked by the markers D6S1714 and D6S1024. We now have refined the genetic interval using new polymorphic markers developed from our BAC/PAC resources. In addition, we have evaluated a recently identified, E]F hand-containing gene that maps to the interval of interest, established its transcript sequence, defined its genomic organization, and excluded this new gene as a PKHD1 candidate. Therefore, this study has narrowed the PKHD1 interval and excluded a potentially relevant gene as a PKHD1 candidate gene. This further refinement of the PKHD1 interval will facilitate efforts to identify the PKHD1 gene by positional cloning. These data also provide additional, highly polymorphic markers for haplotype-based diagnostic testing for ARPKD. © 2002 Wiley-Liss, Inc.