A developer cartridge attachable to a drum unit of an electrophotographic image forming apparatus, comprising: a housing extending in a first direction and configured to contain toner, the housing having a first end in the first direction, and a second end located away from the first end in the first direction; a developing roller rotatable about a roller axis extending in a second direction different from the first direction, the developing roller being located closer to the first end than to the second end in the first direction, the developing roller having an uncovered surface located at a first end of the developing roller in a third direction and exposed outside the housing, and a covered surface located at a second end of the developing roller in the third direction and located inside the housing; a first boss extending in the second direction and movable together with the housing, the first boss being located at a first outer surface of the housing in the second direction, the first boss being located away from the developing roller in the first direction and the third direction; and a second boss extending in the second direction and movable together with the housing, the second boss being located at the first outer surface, the second boss being located farther away from the developing roller than the first boss is from the developing roller in the first direction and the third direction, wherein the second boss is configured to receive a pressing force directed from the second end of the housing in the first direction toward the first end of the housing in the first direction, and wherein, when attached to the drum unit, the developer cartridge is configured to pivot about the first boss in response to the second boss receiving the pressing force directed from the second end of the housing in the first direction toward the first end of the housing in the first direction, wherein the second boss is configured to receive the pressing force directed from the second end of the housing in the first direction toward the first end of the housing in the first direction from a lock lever of the drum unit. a housing extending in a first direction and configured to contain toner, the housing having a first end in the first direction, and a second end located away from the first end in the first direction; a developing roller rotatable about a roller axis extending in a second direction different from the first direction, the developing roller being located closer to the first end than to the second end in the first direction, the developing roller having an uncovered surface located at a first end of the developing roller in a third direction and exposed outside the housing, and a covered surface located at a second end of the developing roller in the third direction and located inside the housing; a first boss extending in the second direction and movable together with the housing, the first boss being located at a first outer surface of the housing in the second direction, the first boss being located away from the developing roller in the first direction and the third direction; and a second boss extending in the second direction and movable together with the housing, the second boss being located at the first outer surface, the second boss being located farther away from the developing roller than the first boss is from the developing roller in the first direction and the third direction, wherein the second boss is configured to receive a pressing force directed from the second end of the housing in the first direction toward the first end of the housing in the first direction, and wherein, when attached to the drum unit, the developer cartridge is configured to pivot about the first boss in response to the second boss receiving the pressing force directed from the second end of the housing in the first direction toward the first end of the housing in the first direction, wherein the second boss is configured to receive the pressing force directed from the second end of the housing in the first direction toward the first end of the housing in the first direction from a lock lever of the drum unit.