ESR1 and breast cancer: These ErbB2 differences between the 3 major dendrogram arms proved statistically significant (Fisher's Exact test, p = 0.03); and in consideration with ER status these 3 groups suggest that they may be generally representative of ErbB2-positive/ER-negative, ErbB2-negative/ER-positive, and ErbB2-positive/ER-positive breast cancers, respectively.