Previously, we reported that sperm-associated antigen 5 (SPAG5; OMIM 615562) is a novel oncogene in estrogen receptor–positive luminal-B subtype breast cancer.8,9 The aim of this study was to analyze the association of SPAG5 gene and SPAG5 protein expression in estrogen receptor–positive breast cancer with treatment response, which may enable better management of estrogen receptor–positive breast cancer. The gene discussed is ESR1; the disease is breast cancer.