A percutaneous transseptal surgical implantation method for replacing a defective atrioventricular (AV) valve with a conical shaped prosthetic valve formed from extracellular matrix (ECM) tissue. When the method is employed to replace a native mitral valve, the method positions the prosthetic tissue valve in the mitral valve region, whereby the valve does not obstruct the outflow tract of the aortic valve and prevents the leaflets of the aortic valve from coapting.