Heparin is an endogenous glycosaminoglycan that is produced by mast cells and basophils and exhibits a negative charge. Heparin displays anticoagulant activity. Heparin binds antithrombin III, activating the protein and inhibiting binding and activation of thrombin and Factor Xa. In vivo, heparin may inhibit the formation of bone and cartilage. Heparin is also a natural ligand for anaplastic lymphoma kinase (ALK), an oncogenic kinase involved in anaplastic large-cell lymphomas.