Isolated cardiac conduction disease is an autosomal dominant defect that includes various combinations of bundle branch or fascicular blocks. These defects can cause sudden death due to a complete heart block. We used a genome-wide screening approach with polymorphic (CA)n repeat markers to determine the chromosomal position of the gene defect implicated in this disorder. The analyses were carried out on a large Lebanese kindred, which included individuals with either a complete or incomplete right bundle branch block (RBBB) with a vertical-axis deviation (< or = -30 or > or = +100). Linkage to the disease locus was detected with the polymorphic marker D19S604 on the q arm of chromosome 19 (19q13.3) with a multipoint lod score of 7.18. Additionally, we were able to exclude the flanking loci D19S606 and D19S571, which are 13 cM apart because of recombination events in three affected individuals. The histidine-rich calcium-binding protein gene is found in this region and is an attractive candidate gene on the basis of its physiological properties and a tight linkage. There is no expansion in two exon 1 regions known for a variable number of triplet repeats.