To detect HER3-specific antibodies that could recognize membrane-associated HER3, binding of HER3-VIA in mouse serum was tested by flow cytometry against a series of human HER3-expressing breast tumor cell lines as targets, including the high HER3-expressing BT474, BT474M1, SKBR3, and T47D, and the HER3 negative, triple negative MDA-MB-231 tumor cell lines. This evidence concerns the gene ERBB3 and breast neoplasm.