ESR1 and breast cancer: Estrogen binding results in recruitment of the basal transcription factor TFIIH through a direct interaction with the ER LBD to promote Ser118 phosphorylation by the cyclin-dependent kinase (CDK)7 kinase of TFIIH.33, 34 We show that CDK7 inhibition is effective in blocking growth of MCF7 ER-Y537S cells, demonstrating the utility of CRISPR-Cas9 generated breast cancer models of ER mutations for evaluating new therapeutic approaches for endocrine resistant breast cancer.