A Long-Term Safety Extension Study of Mavacamten in Adults Who Have Completed MAVERICK-HCM or EXPLORER-HCM

Key Inclusion Criteria: * Has completed the Parent Study through to the EOS Visit within 90 days of consent. (Participants who are beyond the 90-day window from EOS Visit may be included in this study pending MyoKardia Medical Monitoring approval) Participants who prematurely discontinued from the Parent Study or the MAVA-LTE study may be considered for inclusion. * Has a body weight greater than 45 kg at the Screening Visit * Has adequate acoustic windows to enable accurate TTEs. * Has documented LVEF ≥ 50% by echocardiography core laboratory read of screening TTE at rest. * Has safety laboratory parameters (chemistry, hematology, coagulation, and urinalysis) within normal limits (according to the central laboratory reference range). * Female participants must not be pregnant or lactating and, if sexually active, must use one of the following highly effective birth control methods from the Screening Visit through 90 days after the last dose of investigational medicinal product (IMP). In addition to the above contraceptive requirements for female participants, male partners must also use a contraceptive (eg. barrier, condom, or vasectomy). Key Exclusion Criteria: * Has any ECG abnormality considered by the investigator to pose a risk to participant safety (eg. second degree atrioventricular block type II). * Has a history of syncope or a history of sustained ventricular tachyarrhythmia with exercise between Parent Study EOS Visit and Screening Visit. * Has a history of resuscitated sudden cardiac arrest or known history of appropriate implantable cardioverter-defibrillator (ICD) discharge for life-threatening ventricular arrhythmia between Parent Study EOS Visit and Screening Visit. (Note: history of anti-tachycardia pacing (ATP) is allowed).• Currently treated with disopyramide or ranolazine (within 14 days prior to Screening) or treatment with disopyramide or ranolazine is planned during the study. * Has any acute or serious comorbid condition (eg. major infection or hematologic, renal, metabolic, gastrointestinal, or endocrine dysfunction) that, in the judgment of the Investigator, could lead to premature termination of study participation or interfere with the measurement or interpretation of the efficacy and safety assessments in the study. * History of clinically significant malignant disease that developed since enrollment in the Parent Study. * Is unable to comply with the study requirements, including the number of required visits to the clinical site. * Has participated in a clinical trial in which the participant received any investigational drug (or is currently using an investigational device) within 30 days prior to Screening, or at least 5 times the respective elimination half-life (whichever is longer), except for participation in MAVERICK-HCM or EXPLORER-HCM. Prior participation in a non-interventional observational study is allowed.