The invention relates to a system for treating a native mitral valve, comprising, a delivery catheter (124)； a prosthetic mitral valve assembly (260) comprising a self-expanding stent (10) and a valve (18) made from pericardial tissue, the valve mounted within the stent (10) and forming collapsible leaflets (62) for replacing the function of the native mitral valve, the stent having a substantially D-shape cross-section configured to conform to the natural opening of the native mitral valve, the stent including an upper portion (12) having an enlarged or flared end (14) that tapers to a lower portion (16) having a reduced diameter, the enlarged or flared end (14) being sized to be positioned above an annulus (30) of the native mitral valve (24)； and a tether (80) coupled between a lower portion of the prosthetic mitral valve assembly and a wall of the heart for preventing upward movement of the prosthetic mitral valve assembly (260)； wherein the prosthetic mitral valve assembly (260) is adapted to be advanced in a radially compressed state along a distal end of the delivery catheter (124) for insertion through the wall of the heart.
