The present invention relates to a plunger rod resetting mechanism for a drug delivery device comprising a housing (12) having an outer surface on a proximal end configured to allow removable attachment of a cartridge holder (200) through engagement of a snap fit connector (211) on the outer surface a locking shell (205) positioned inside the housing and having a radial projecting pin protruding through the outer surface of the housing (12), where the locking shell (205) is rotatable relative to the housing (10) and configured to engage the cartridge holder (200) during attachment to the housing (10) a lock pin (214) rotationally fixed to the housing and having a stop (215) projecting through the outer surface of the housing (10) and slidable in the distal direction from an unlocked to a locked position where the stop (215) prevents removal of the cartridge holder (200) from the housing a biasing member (204) that exerts a distal force on the lock pin (214) to move the stop (215) from the unlocked to the locked position a transfer ring (216) rotationally fixed to and axially slidable to the locking shell (205) a cam nut (226) engaged with the transfer ring (216) through a cam that allows the transfer ring (216) to rotate and move axially relative to the cam nut (226) a push pin (217) axially fixed to the transfer ring (216) and a nut segment (218) slidably connected to the push pin (217) through a cam, where the nut segment (218) is configured to move from a locked position to an unlocked position.
