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Inclusion Criteria:

• Signed informed consent form (ICF): Participants, or their legally acceptable representative, must have signed and dated an Institutional Review Board (IRB)/Independent Ethics Committee (IEC)-approved ICF in accordance with regulatory, local, and institutional guidelines. This must be obtained before the performance of any protocol-related procedures.

* Diagnosed with obstructive hypertrophic cardiomyopathy (HOCM) consistent with Japanese Circulation Society guidelines (2025), i.e., satisfy all criteria below:

  * Has unexplained left ventricular (LV) hypertrophy with nondilated ventricular chambers in the absence of other cardiac (e.g., hypertension, aortic stenosis) or systemic disease and with maximal LV wall thickness ≥ 15 mm (or ≥ 13 mm with positive family history of HCM).
  * Has Left Ventricular Outflow Tract (LVOT) peak gradient ≥ 30 mmHg (resting, Valsalva maneuver, or post-exercise).
* Has documented Left Ventricular Ejection Fraction (LVEF) ≥ 55% at baseline.
* Participants who meet any of the following criteria:

  * Participants who have previously received mavacamten continuously for ≥ 16 weeks
  * Participants who are currently receiving mavacamten
  * Participants who are scheduled to receive mavacamten
* Treated with a stable dose of cibenzoline for at least 3 months prior to initiating mavacamten treatment. Tapered cibenzoline within 3 months prior to initiating mavacamten treatment is allowed if stable dose of cibenzoline was used for at least 3 months prior to tapering.
* At least 18 years of age at the time of signing the informed consent.

Exclusion Criteria:

* Hypersensitivity to the active substance or to any of the excipients.
* During pregnancy and in women of childbearing potential.
* Treated with strong CYP3A4 inhibitors (itraconazole, clarithromycin, voriconazole, posaconazole, ritonavir, cobicistat, ceritinib, ensitrelvir fumaric acid, lonafarnib, josamycin, or mifepristone/misoprostol).
* Severe hepatic impairment (Child-Pugh C).
* Severe atrioventricular block or severe sinoatrial block.
* Congestive heart failure.
* Requiring dialysis.
* Angle-closure glaucoma.
* Tendency to urinary retention.
* Treated with vardenafil hydrochloride hydrate, moxifloxacin hydrochloride, lascufloxacin hydrochloride (injection), toremifene citrate, fingolimod hydrochloride, siponimod fumarate, or eliglustat tartrate.
* Mavacamten treatment within 8 weeks prior to baseline. Mavacamten treatment initiation was judged based on post-exercise LVOT peak gradient.

• Minato-ku, Tokyo, Japan