Brief Summary: Prospective, single-arm, multicenter study to evaluate the safety and performance of the AltaValve for the treatment of moderate to severe or severe mitral valve regurgitation in subjects who are considered high risk for mortality and morbidity from conventional open-heart surgery. Inclusion Criteria: Subjects = 18 years of age. Subjects symptomatic New York Heart Association (NYHA) II-IV. Subjects with severe MR as documented by echo. Subjects who are at high risk for open-heart surgery as documented by the health care professional (e.g., Heart Team consisting of cardiac surgeon and interventional cardiologist in United States). Abbreviated Exclusion Criteria: Inability to understand the study or a history of non-compliance with medical advice. Unwilling or unable to sign the Informed Consent Form (ICF). History of any cognitive or mental health status that would interfere with study participation. Currently enrolled in any other pre-approval investigational study (does not apply to long-term post-market studies unless these studies might clinically interfere with the current study endpoints (e.g., limit use of study-required medication, etc.)). Female subjects who are pregnant or planning to become pregnant within the study period. Known hypersensitivity or contraindication to aspirin, heparin, or Warfarin without adequate alternative medications. Known hypersensitivity to nitinol (i.e., nickel allergy) that cannot be adequately medicated. Known hypersensitivity to contrast media that cannot be adequately medicated. Evidence of current Left Ventricular Ejection Fraction (LVEF) =30% (where current is defined as the latest LVEF measurement completed within 90 days prior to the index procedure). Concurrent medical condition with a life expectancy of less than 12 months. Prior mitral valve repair/replacement (excluding prior surgical mitral valve repair, annuloplasty, or MitraClip not interfering with AltaValve placement).