A developer cartridge comprises a first gear. The first gear may include a first protrusion extending in a radial direction of the first gear. The first protrusion may be positioned at a circumference of a column of the first gear. The first protrusion may be spaced apart from a second end face opposite to a first end face facing an exterior surface of a casing configured to accommodate developer therein. The first protrusion may be further from the second end face than a large-diameter gear in an axial direction. A rotational circumference of the first protrusion defined by rotation of the first protrusion and a portion of the large-diameter gear are aligned in the axial direction.