The mesylate salt of obatoclax, a synthetic small-molecule inhibitor of the bcl-2 family of proteins with potential pro-apoptotic and antineoplastic activities. Obatoclax binds to members of the Bcl-2 protein family, preventing the binding of these anti-apoptotic proteins to the pro-apoptotic proteins Bax and Bak and so promoting the activation of the apoptotic pathway in Bcl-2-overexpressing cells. The Bcl-2 family of proteins (bcl-2, bcl-xl, bcl-w, and Mcl-1) are overexpressed in a wide variety of cancers, including those of the lymphatic system, breast, lung, prostate, and colon.