ERBB2 and breast cancer: Moreover, in luminal HER2-negative BC cell lines that ectopically expressed the human d16HER2 or wtHER2 isoform, based on the efficiency of colony formation—an in vitro assay that tests the stemness of cancer cell lines [56]—the number and size of d16HER2-positive colonies rose significantly versus wtHER2-positive colonies [24].