Using the TCGA breast cancer dataset (n = 282 EXO1-overexpressing ER-positive cases, ∼20,000 genes per sample), we stratified tumors into high- and low-HRD score groups based on a widely used cutoff value of 42, which has been established as a clinically relevant threshold for defining HR deficiency (Choi and Lee, 2022). The gene discussed is ESR1; the disease is breast cancer.