ERBB2 and carcinoma: While the lack of cellularity was unlikely to reflect an absence of tumor tissue after a single cycle of HER2-targeted therapy, interestingly, an on-treatment biopsy without carcinoma was modestly predictive of pCR (OR 2.5 (95% CI 1.0–6.5)), suggesting biologically relevant patchiness of tumor occurring very early in treatment, though this variable was not significant when added to our base multivariate model of pCR (Supplementary Table 3), as absence of tumor was more common in the combination arm (χ2 p = 0.0079).