AXL and breast cancer: For developing an assay to detect single AXL+ CTCs, two commercially available breast cancer cell lines were used to determine the sensitivity, specificity, and dynamic range of two anti‐AXL antibodies (MAB154R antibody coupled to Alexa Fluor 555 and FAB‐154P) by IF analysis and flow cytometry, and then in the CellSearch® system (Figure 1A–C).