ERBB2 and breast cancer: In a spiking experiment with 10, 100 and 1000 tumor cells of three breast cancer cell lines with different subtypes (SKBR3 hormone receptor (HR)-negative/HER2-positive, MCF-7 HR-positive/HER2-positive, Hs578T HR-negative/HER2-negative) into HD blood, the efficiency of the individual markers in combination with the CTC isolation with Histopaque density gradient centrifugation was evaluated (Figure S1).