As can be seen from the immunofluorescence results, HER2-positive cells were also visualized in paraffin sections of the brain (Fig. 4D) and liver (Fig. 4E), demonstrating that sdAb-HER2-QDs have a high potential as imaging probes for the detection of metastatic HER2-expressing breast tumour cells in 50-μm tissue sections. This evidence concerns the gene ERBB2 and breast neoplasm.