Amyotrophic lateral sclerosis (ALS) denotes a heterogeneous group of neurodegenerative disorders affecting upper and lower motor neurons. ALS4 is a juvenile-onset, autosomal dominant form of ALS that is characterized by slow progression, distal limb weakness and amyotrophy, and pyramidal signs associated with severe loss of motor neurons in the brain and spinal cord. The ALS4 locus was recently mapped by linkage analysis to a large genetic interval on chromosome 9q34. By undertaking extensive genetic linkage analysis, we have significantly refined the ALS4 locus to a critical interval of less than 3 cM, flanked by D9S149 and D9S1198. Previous physical mapping in this region has indicated that this critical interval spans approximately 500 kb. Seventeen putative transcripts have been localized within this interval including 7 characterized genes, 2 partially characterized genes, and 8 "anonymous" expressed sequence tags. These are therefore positional candidate genes for the ALS4 locus. We have also undertaken mutation analysis and genetic mapping to investigate and exclude candidate genes, including RING3L/ORFX and RALGDS, from a pathogenic role in ALS4.