In this study, clustered regularly interspaced short palindromic repeats-CRISPR-associated protein 9 (CRISPR-Cas9) generation of ER+BC cell line MCF7 with a gnomically encoded Y537S mutation in the estrogen receptor alpha (ESR1) gene was used [25], and patient-derived xenograft models in which the patient developed the Y537S or D538G mutations were used. Here, ESR1 is linked to breast cancer.