ESR1 and breast cancer: We here provided novel insight into the paradox of PR action showing that in spite of a clear function of PR as a major driver of mammary tumor growth, its role as a marker of good prognosis may be explained by our demonstration that in breast cancer cells sensitive to endocrine therapy with Tam, PR must interact with unliganded ERα to exert its rapid and genomic effects leading to the assembly of transcriptional complexes which govern breast cancer growth.