A Phase 2 Study of Mavacamten in Adults With Symptomatic Non-Obstructive Hypertrophic Cardiomyopathy (nHCM)

Key Inclusion Criteria: * Diagnosed with nHCM (hypertrophied and non-dilated left ventricle in absence of systemic or other known cause), with LV wall thickness ≥ 15mm at Screening or ≥ 13mm with a positive family history of HCM. * Age 18 and greater, Body weight \> 45kg * Documented LVEF ≥ 55% at the Screening as determined by echo central lab * LVOT gradient \< 30 mmHg at rest AND during Valsalva AND post-exercise * NYHA functional class II or III * Elevated NT-proBNP at rest Key Exclusion Criteria: * History of syncope, sustained ventricular tachyarrhythmia with exercise, obstructive coronary artery disease or myocardial infarction within the past 6 months * History of resuscitated sudden cardiac arrest at any time or known appropriate implantable cardioverter defibrillator (ICD) discharge within 6 months prior to Screening * Current treatment with disopyramide or ranolazine (within 14 days prior to Screening) * Current or planned treatment during the study with a combination of beta-blockers and calcium channel blockers * Has been treated with invasive septal reduction (surgical myectomy or percutaneous alcohol septal ablation \[ASA\]) within 6 months prior to Screening * History of resting or post-exercise LVOT \>30 mmHg unless subsequently treated by septal reduction * Has QTc Fridericia (QTcF) \>480 ms or any other ECG abnormality considered by the investigator to pose a risk to participant safety (eg, second-degree atrioventricular block type II) * Has persistent or permanent atrial fibrillation not on anticoagulation for at least 4 weeks prior to Screening and/or not adequately rate-controlled within 1 year of Screening * History of clinically significant malignant disease within 10 years such as non-metastatic cutaneous squamous cell or basal cell carcinoma * History or evidence of any other clinically significant disorder, condition, or disease that, in the opinion of the investigator or MyoKardia physician, would pose a risk to subject safety or interfere with the study evaluation, procedures, or completion.