A gene transcription signature of ER- (hormone-independent) versus ER+ (hormone-dependent) invasive breast cancer (IBC) was defined by selecting genes showing differential expression (p<0.01) in each of two independent mRNA profile datasets of 295 clinical IBC (the dataset from van de Vijver et al., ref [9], with 69 profiles from ER- tumors) and 286 tumors (the dataset from Wang et al., ref [10], with 77 ER- profiles). This evidence concerns the gene ESR1 and invasive breast carcinoma.