The prevalence of congenital heart defects is approximately 1% of all live births. Identifying the genes responsible for cardiac malformation is the first step to understand pathogenesis. Heterozygous mutations in the CSX/NKX2-5 (NKX2E) gene have been identified to cause atrial septal defect (ASD) and/or atrioventricular (AV) conduction disturbance in some families. However, there is great variability in expressivity of the phenotype between the patients with a CSX/NKX2-5 mutation. We screened four sporadic patients and three index cases of families with ASD and/or conduction defects. In one of them, a CSX/NKX2-5 mutation was identified. This novel mutation (p.Tyr256X) was inherited in a three-generation family causing five individuals to have cardiac anomalies ranging from ASD to arrhythmias. Interestingly, all the observed AV conduction disturbances were at the nodal level, manifesting first as an AV block of the first degree and evolving toward a second-degree block. Atrial fibrillation, previously reported in three individuals with CSX/NKX2-5 mutations, was observed in three patients.