ERBB2 and breast carcinoma: When HER2-low tumors were further divided into HER2 IHC1+ and HER2 IHC2+ tumors, there were significant differences in HR status (p < 0.001), primary breast cancer diagnosis stage (p < 0.001), performance status (p = 0.002), histological grade (p = 0.015), and number of metastatic sites (p = 0.03) among HER2-zero, HER2 IHC1+, and HER2 IHC2+ tumor groups (Supplementary Table S1).