CA9 and breast cancer: In order to investigate whether a combination of two fluorescent VHHs specifically recognizing two separate and validated breast cancer biomarkers could (a) improve tumour detection through optical imaging with an increase in T/B ratio and/or (b) facilitate tumour characterization and observation of different areas of the tumour, we used the HER2-specific VHH, 11A4 [7], and the CAIX-specific VHH, B9 [11].