A Study About the Natural History in Adults With BAG3 Dilated Cardiomyopathy (a Type of Heart Disease) (BAG3 DCM)

The purpose of this study is to learn about the natural progression of DCM (dilated cardiomyopathy) caused by BAG3 gene mutations. DCM is a condition as the heart muscle is weakened and the heart becomes enlarged. This makes it hard for the heart to pump enough blood for the body. The study is seeking up to about 35 participants who have: * BAG3 mutation (change in the gene) that causes or is likely to cause dilated cardiomyopathy * NYHA (New York Heart Association) Class I-IV at screening (Stage B-D) * Left Ventricular Ejection Fraction less than or equal to 50% (meaning reduced heart function) All participants in this study will receive their usual treatment. The investigators will observe the natural progression of people who have BAG3 DCM. This will help the investigators better understand the disease and aid in future research. Participants will take part in this study for three years. During this time, participants will visit the site at least 8 times (about every 3 months for the first year and annually during year 2 and three). Participants will undergo study procedures and give information about their health. These procedures will include a physical exam, cardiac magnetic resonance imaging, echocardiography, ECG monitoring, activity monitoring, cardiopulmonary exercise testing, and blood tests. Participants will answer questions about health and quality of life. The study team will also call participants about 1 time over the phone.