Tumors with high ERBB2/CEP17 copy number ratio (≥6) had significantly greater cellular heterogeneity than ones with low (<6) in the pretreatment but not the posttreatment setting, suggesting that T-DM1+P treatment may preferentially target tumor cells with higher ERBB2 copy number gain (Supplemental Figure 4C). Here, ERBB2 is linked to neoplasm.