Retrospective  Inclusion Criteria:  Data is available for patient <18 years of age. Documented results of genotyping showing the presence of at least one pathogenic or likely pathogenic MYBPC3 mutation (heterozygous, homozygous, or compound heterozygous).  Exclusion Criteria:  Patient received cardiac transplantation or died >10 years before study initiation.  Prospective  Inclusion Criteria:  Age <18 at entry into the prospective study. Documented results of genotyping identifying at least one pathogenic or likely pathogenic MYBPC3 mutation (heterozygous, homozygous, or compound heterozygous). Infants who are homozygous or compound heterozygous for the known pathogenic truncating MYBPC3 mutations are eligible. Heterozygous participants must have a diagnosis of Cardiomyopathy (CM: HCM, DCM, RCM, mixed CM, or LVNC.  Exclusion Criteria:  Concurrent participation in an interventional clinical trial unless approved by the sponsor. Severe noncardiac disease is anticipated to significantly reduce life expectancy.