ERBB2 and breast cancer: For patients with HER2-low from HER2-zero breast cancers, there were 494 lesions (369 HER2-low and 125 HER2-zero) in the training set, 213 lesions (166 HER2-low and 47 HER2-zero) in the internal validation, and 122 lesions (93 HER2-low and 29 HER2-zero) in the external validation set (Figure 1).