ESR1 and cancer: Analysis of mRNA-seq data from 1,177 primary breast cancers in the cancer genome atlas (TCGA) revealed that the RET mRNA expression level was highest in ER+ breast cancer and correlates positively with the expression level of ESR1 (ERα) (Spearman’s ρ = 0.51, p < 2.2e-16; Fig 5A), suggesting that it is a direct transcriptional target of ERα in vivo as well.