Researchers at Penn Medicine have made a major advance in understanding the biology of a common, puzzling, and often fatal heart disorder, dilated cardiomyopathy (DCM), which features the enlargement of the heart and a progressive decrease in its function, for reasons other than cardiovascular disease. DCM is estimated to affect at least hundreds of thousands of people in the United States. The largest single known cause, accounting for an estimated 10 to 20 percent of cases, involves the mutation of the gene that encodes a key heart-muscle protein called titin.