Significantly, the presence of residual elafin-positive cells was a strong, independent prognostic marker of poor recurrence-free survival (RFS) specifically in IHC-defined luminal A subtype tumors (that is estrogen receptor-positive (ER+)/progesterone receptor-positive (PR+)/Ki67low), the largest and most diverse breast cancer subtype and the greatest contributor to overall breast cancer mortality [11]. This evidence concerns the gene ESR1 and breast cancer.