ESR1 and cancer: Amplifications of MCL1 (1q21.2) and NTRK1 (1q23.1) are more prevalent in ER-positive/HER2-negative/low proliferation/luminal A cancers than in ER-positive/HER2-negative/low proliferation/claudin-low cancers (Fisher’s exact test p = 0.0001 and 0.001, respectively, Table 9), while other commonly amplified loci in breast cancer do not show significant differences in the two groups, despite being numerically more prevalent in claudin-low cases.