MM-111 is a bispecific antibody fusion protein targeting the oncogenic units ErbB2/ErbB3. It obstructs the activation of the PI3K survival pathway. By binding to the ErbB2 receptor, MM-111 localizes the bispecific molecule to ErbB2-overexpressing tumor cells, facilitating the interaction of its anti-ErbB3 arm with the ErbB3 receptor. This binding inhibites ErbB3 signaling by preventing the ErbB3 receptor from associating with its physiological ligand, heregulin. MM-111 is applicable in the study of ErbB2-overexpressing breast tumors.