A prosthesis delivery system comprises a sheath defining an axial direction; at least one tether movable axially relative to the sheath; and a holder movable axially within the sheath, configured to constrain radially the tether(s). Collapsing the prosthesis to a collapsed state in the sheath involves constraining radially with a holder the tether(s), attached to the prosthesis; and moving the tether(s) and the holder axially within the sheath such that the prosthesis is forced to collapse into the sheath and the holder is retracted into the sheath.