1. Clarification of the visit information for subjects that may require a longer active-run phase; 2. Clarification of the visit information for children ≤16 years and <35 kg in the active-run; 3. Clarification of the dose adjustment guidelines for children who reach 17 years of age or whose body weight increases from <35 kg to ≥ 35 kg in the maintenance phase; 4. Clarification of the adverse event reporting timeframe to the DCC; 5. Clarification of the acquisition and handling of blood samples; 6. Clarification of the roles for the management of the investigational drugs; and 7. Clarification of the potential risk that valsartan administration may contribute to the development of obstructive physiology.

1. Change in nomenclature to clarify that subjects designated as overt HCM may have borderline left ventricular hypertrophy and not meet standard clinical criteria for a diagnosis of HCM. The cohort previously designated "overt HCM" will now be designated "Group 1". The cohort previously designated as "preclinical HCM" will now be designated "Group 2". 2. Section 5.2: Clarification of Inclusion Criteria: All Group 1 subjects regardless of age may be eligible based on either LV wall thickness in mm ( >/= 12 mm and </= 20 mm) or z score (>/= 3 and </=14) criteria. Group 1 z-score cap should be </=14 (not 10) to correlate with LVWT 20 mm. 3. Section 5.3: Subject Exclusion Criteria Addition of concomitant or prior use of Spironolactone, Lithium, or Aliskiren 4. Section 5.3: Subject Exclusion Criteria- Clarification prior ARB/ACE-use is permitted if medications have been discontinued or can be discontinued/ not needed for clinical care. A minimum 2-week washout period should be performed before proceeding to baseline studies. 5. Section 5.3: Subject Exclusion Criteria - Clarification of echocardiographic core lab eligibility 6. Section 5.3: Subject Exclusion Criteria - Clarify the exclusion for hypertension to exclude only those with uncontrolled, persistent hypertension (SBP>160 and/or DBP>90) 7. Section 7.7: Clarification of FitBit distribution 8. Section 8.2.2: Restrictions regarding concomitant treatment Addition of Spironolactone, Lithium, or Aliskiren to the list of restricted medications. 9. Section 9.1: Recording and Reporting of Adverse Events - Modification to remove independent medical monitor adjudication. 10. Section 10.4: Adjudication Procedures- Clarification of the process for adjudicating clinical events 11. Section 22.1 Appendix 1: Participating Sites Modifications to update the participating sites and investigators 12. We would like to expand our internet advertising strategy to include social media. We have included Face

1. Minor grammatical/style changes were made throughout 2. Page 5, Section 1.2: Protocol Synopsis • Edited to reflect changes in eligibility (listed below) • Edited to reflect that baseline studies are considered current for up to six months 3. Page 14, Section 4.1 Trial Schema • Figure 4 updated to reflect changes in eligibility (listed below) 4. Page 15, Section 5.1: Subject Selection • Edited to reflect changes in group 2 eligibility (listed below) 5. Page 15-16, Section 5.2: Amendment of Inclusion Criteria • Genetic testing not performed by a laboratory not listed in the inclusion criteria will be reviewed by the Clinical Coordinating Center to determine eligibility. • The upper age range for Group 1 subjects will be increased from 30 to 45 years • The maximal LV wall thickness for Group 1 subjects will be increased from 20 to 25 mm or a z-score of 18 Rationale: Experience during the first 6 months of study enrollment indicated that the target population for this trial, as defined by original inclusion/exclusion criteria, is exceedingly rare. Even in the specialty HCM centers participating in the trial, the target population represents a very low prevalence group. As such, we recognize the need to modify eligibility criteria to facilitate enrollment. After detailed review of site screening logs, it was determined that allowing inclusion of subjects up to age 45 years, subjects with a maximal LV wall thickness of up 25mm, and subjects with primary prevention ICD devices will substantially increase the yield of eligible subjects without compromising study safety and design. • A LV wall thickness z-score of 1.5-2.9 combined with LV thickness to dimension ratio ≥0.19 (as determined by rapid assessment by the echocardiographic core laboratory) will qualify subjects for Group 2 Rationale: The combination of an upper normal LV wall thickness combined with an increased thickness:dimension ratio is anticipated to accurately discriminat

Added additional performance sites

The protocol has been updated in the following sections: -Section 1.2 - Protocol Synopsis -Section 6.1 - Subject Participation -Section 8.4 - Procedures for Monitoring Subject Compliance These changes relate to patients that have their Year 2 visit out of window and the timeframe in which they continue taking study drug. The protocol changes indicate that patients will continue taking study medication up until their Year 2 visit, even if the visit occurs out of window. If a patient has an out of window Year 2 visit, the patient should continue study medication until the visit occurs. This is to ensure that Year 2 studies are performed on therapy and preserve the reliability/interpretability of their results.