A transluminal sheath is advanced transseptally into a left atrium of the subject. A distal end of a surrounding-sheath, having an anchor disposed therein, is advanced via a distal end of the transluminal sheath, into a left ventricle of the subject via a commissure of the mitral valve. While the distal end of the surrounding-sheath is in the left ventricle, the surrounding-sheath is pulled proximally with respect to the anchor to expose the anchor. While the distal end of the surrounding-sheath is in the left ventricle, mitral valve tissue that is within the left ventricle is encircled by helically wrapping the anchor around the mitral valve tissue. Subsequently, the surrounding-sheath is extracted from the heart. Other embodiments are also described.