Our recent studies discovered that AXL, a receptor protein tyrosine kinase (RTK), is selectively overexpressed and activated in highly aggressive TNBC cells. Based on this discovery in this study, we propose to establish AXL as a novel therapeutic target in the treatment of patients with TNBC. To achieve this, we will develop a novel therapeutic agent, a humanized anti-AXL ADC to specifically and effectively target TNBC with AXL over expression. Specific Aims: Aim 1. To develop a humanized antibody-drug conjugate (ADC) that can effectively target the AXL membrane receptor tyrosine kinase. Aim 2. To test the efficacy of the AXL-targeted ADC in preclinical animal models Aim 3. To develop a mass spectrometry-based method to effectively monitor AXL expression and activation in xenograft tissues and clinical samples.