The deltoid tuberosity is a superstructure on the humerus and is a reliable readout of tendon and enthesis development as well as muscle loading in the embryonic limb. In ScxCre-Sox9 mutant mouse embryos (top row), the deltoid tuberosity (black arrow) and triceps insertion (purple arrow) fail to initiate formation. In muscleless mouse embryos, the formation of the deltoid tuberosity is initiated at embryonic day (E) 14.5; however, without muscle contraction, the tuberosity is not maintained by E18.5. Images modified with permission from (23,27).