This is a multicenter, exploratory, randomized, double-blind study of the administration of mavacamten in 60 participants with symptomatic nHCM randomized to receive a 16-week course of mavacamten doses titrated to achieve 1 of 2 target drug concentrations.

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.