ERBB2 and breast cancer: When we subdivided HER2-low breast cancers into HER2 1+ and HER2 2+ ISH– tumors, we found a higher proportion of non-basal-like tumors in the HER2 2+ subgroup than in the HER2 1+ subgroup (57.1% vs. 17.8%, P = 0.003, Fisher’s exact test) (Fig. 4b).