To determine whether the prognostic power of the signature could be extended to ER+ breast cancer, we investigated the activity of the MaSC gene signature in ER+ tumours of three different breast cancer cohorts, namely the NKI295 dataset (226 ER+ tumours/295 samples) [28], the TRANSBIG dataset (134 ER+ tumours/198 samples) [29] and the GSE2990 dataset (149 ER+ tumours/189 samples) [30]. This evidence concerns the gene ESR1 and breast cancer.