ESR1 and breast cancer: Furthermore, ESR1 and CYP19A1 appear to co-amplify cooperatively in AI-treated patients as it was found that 32.5% of AI-treated patients had CYP19A1 amplification, 21.5% presented with ESR1 amplification, and approximately 66% of breast cancers with CYP19A1 amplification showed ESR1 co-amplification (213).