Familial dilated cardiomyopathy (DCM) is a heterogeneous disease; although 30 disease genes have been discovered, they explain only no more than half of all cases; in addition, the causes of intra-familial variability in DCM have remained largely unknown. In this study, we exploited the use of whole-exome sequencing (WES) to investigate the causes of clinical variability in an extended family with 14 affected subjects, four of whom showed particular severe manifestations of cardiomyopathy requiring heart transplantation in early adulthood. This analysis, followed by confirmative conventional sequencing, identified the mutation p.K219T in the lamin A/C gene in all 14 affected patients. An additional variant in the gene for titin, p.L4855F, was identified in the severely affected patients. The age for heart transplantation was substantially less for LMNA:p.K219T/ TTN:p.L4855F double heterozygotes than that for LMNA:p.K219T single heterozygotes. Myocardial specimens of doubly heterozygote individuals showed increased nuclear length, sarcomeric disorganization, and myonuclear clustering compared with samples from single heterozygotes. In conclusion, our results show that WES can be used for the identification of causal and modifier variants in families with variable manifestations of DCM. In addition, they not only indicate that LMNA and TTN mutational status may be useful in this family for risk stratification in individuals at risk for DCM but also suggest titin as a modifier for DCM. With the exception of TTN, truncating mutations of which have been found in up to 27% of individuals with DCM, 1 a single DCM-causing gene accounts for no more than 6-8% of all cases. The known DCM disease genes include those encoding for proteins of the sarcomere, the Z-disk, the cytoskeleton, the mitochondria, RNA binding proteins, the sarcoplasmic reticulum, and the nuclear envelope. 2-4 Familial DCM exhibits a remarkable degree of clinical variability with respect to severity, penetrance, and age of onset. 5 Like familial hypertrophic cardiomyopathy (HCM), familial DCM is often characterized by incomplete penetrance, a high degree of variable expressivity even among