A subclass of estrogen receptor-negative (ER−) tumors that overexpress IR genes and that have a good prognosis compared with the rest of ER− breast tumors independently of lymph node status or lymphocytic infiltration was identified. Subsequently, an associated module of complement and IR genes that define prognostic markers was identified and validated in over 240 ER− samples. This evidence concerns the gene ESR1 and breast neoplasm.