Since CXXC5 is an E2-ERα responsive gene and the CXXC5 protein appears to be critical for E2-mediated cellular events in MCF7 cells modeling ER-positive breast cancer, we wanted to assess whether there is a correlation between the CXXC5 and ESR1 (ERα) expressions and whether the CXXC5 expression has a prognostic value in ER+ breast cancer patients using two large, publically available breast cancer patient datasets, METABRIC (Molecular Taxonomy of Breast Cancer International Consortium) and TCGA (The Cancer Genome Atlas). The gene discussed is ESR1; the disease is breast cancer.