HALO: A Single Arm Prospective Investigation of the SJM™ Masters HP™ 15mm Rotatable Mechanical Heart Valve

IDE Cohort - Inclusion Criteria: 1. Subject requires mitral valve replacement.\* 2. Subject's legally authorized representative gives written consent to participate in the clinical study. 3. Subject is willing and able to return for data collection and follow-up for the duration of the clinical study. * Subjects undergoing concomitant procedures (e.g. valve repair) are eligible for this study other than those noted in the exclusion criteria. Exclusion Criteria: 1. 1\. Subject is \> 5 years of age. 2. Subject has a contraindication to anticoagulant/antiplatelet medication. 3. Subject has a prosthetic valve(s) at a site other than the mitral valve prior to the study procedure.\* 4. Subject requires concomitant replacement of the tricuspid, pulmonary, or aortic valve. 5. Subject has active endocarditis. 6. Subject has active myocarditis. 7. Subject has had an acute preoperative neurological deficit that has not returned to baseline or stabilized ≥ 30 days prior to the study procedure. 8. Subject has had an acute cardiac adverse event that has not returned to baseline or stabilized ≥48 hours prior to the study procedure. 9. Subject has a non-cardiac illness resulting in a life expectancy of \< 1 year. 10. Subject has a known requirement for additional cardiac surgery within 12 months after the study procedure. 11. Subject has been previously enrolled and implanted in this study. 12. Subject is participating in another study for an investigational drug and/or device. 13. Subject has any other medical condition that in the opinion of the Investigator will interfere with the study results. * Subjects who have undergone a previous Ross procedure of the pulmonary valve are eligible for this study. Continued Access Cohort- Prospective Inclusion Criteria: 1. Subject requires mitral valve replacement.\* 2. Subject's legally authorized representative gives written consent to participate in the clinical study. 3. Subject is willing and able to return for data collection and follow-up for the duration of the clinical study. * Subjects undergoing concomitant procedures (e.g. valve repair) are eligible for this study. Prospective Exclusion Criteria: 1. Subject is \> 5 years of age. 2. Subject has a contraindication to anticoagulant/antiplatelet medication. Retrospective Eligibility Criteria: 1. The subject must have been ≤ 5 years of age at the time of mitral valve replacement with the 15mm MHV. 2. An implant was attempted with the 15mm MHV, where implant attempt is defined as the device physically contacting the subject's cardiac anatomy. 3. The legally authorized representative signs the study informed consent for this protocol allowing access to all relevant historical medical information and prospective follow-up (if applicable). 4. Either 1. the legally authorized representative and site agree to follow the subject per the assessment schedule and complete all required assessments per this protocol from the time of consent going forward. OR 2. the subject's status is deceased or explanted, but an implant with 15mm MHV was attempted.