ERBB2 and breast carcinoma: HER2 protein expression as quantitated by immunoblot was not, however, significantly elevated in models with lower levels of ERBB2 copy number gain versus PDO models wildtype for ERBB2. These data suggest that the threshold employed clinically for identifying urothelial tumors with ERBB2 amplification likely overstates the frequency of amplification mediated HER2 protein overexpression and are consistent with the lower degree of correlation between ERBB2 amplification and HER2 overexpression noted in urothelial versus breast cancers in the TCGA cohort (Supplementary Fig. 3C).