ERBB2 and breast cancer: Consequently, we measured the impact of patient sera exposure on mammary tumor cells isolated from MMTV-neu mice, which also overexpress HER2, and found that ESR2 mRNA levels were 67% (± 11.4%) higher (p<0.01) and ERβ protein levels were 44% (± 3.0%) greater (p<1x10-5) in cells exposed to N versus OB patient sera (Fig 1A and 1B).