Connexin43 is one of connexin proteins which make up the intercellular gap junctions. Targeted null mutation of the mouse connexin43 gene has been reported to result in a cardiac malformation. Moreover, single-base mutations of the human homolog (GJA1) were identified in patients with laterality defects of the chest and abdominal organs, suggesting that connexin43 contributes to the determination of laterality during organogenesis. We mapped GJA1 to 6q22.3 by fluorescence in situ hybridization, using a bacterial artificial chromosome (BAC) clone that covered almost the entire GJA1-cDNA, as a probe.