From several familial cases of bundle branch system defect (cardiac conduction disease) collected by Pr E. Stephan (Université St Joseph, Beirut, Lebanon), a region ranging from markers D19S879 to D19S866 (excluded) from the 19q13.3 fragment of chromosome 19 has been implicated. Further localisation, made by the Dr Bouvagnet group (Montpellier, France), restricted the target to cosmid 34439, which has been obtained from Lawrence Livermore National Laboratory.