Inclusion Criteria: * Pathogenic or likely pathogenic mutation in BAG3 * Medical history of diagnosis of DCM * Stable combination of HF SoC medications * Adequate acoustic windows for echocardiography * LVEF \> 15% and ≤ 45% at Screening by echocardiography * Diagnosis of chronic HF for at least 3 months * NYHA Class I/II/III at Screening Exclusion Criteria: * Presence of antibodies to AAV9 * Presence of a pathogenic or likely pathogenic variant in another gene where that other gene is authoritatively recognized as causal for DCM * Decompensated HF * Cardiac ventricular arrhythmia that requires treatment * History of untreated clinically significant valve disease or a Screening confirmation of severe aortic stenosis, severe mitral stenosis, moderate or severe aortic insufficiency or severe mitral insufficiency * NYHA Class IV * History of substance abuse that could contribute to cardiac dysfunction Note: Other inclusion and exclusion criteria may apply.