For CTC enumeration purposes, we considered as CTCs those cells presenting with cytokeratin (CK)-positivity, CD45-negativity and cancer-related morphological features (e.g. large nucleolus, irregular size and shape, scarce cytoplasm), plus at least one of the following parameters: presence of multicellular aggregates of CK-positive cells (at least two cells), positivity for the oestrogen receptor (ER) or gene amplification of the tyrosine kinase receptor ErbB2 (HER2, evaluated by fluorescence in situ hybridisation). Here, ERBB2 is linked to cancer.