ESR1 and endometrial cancer: The patient tissue and corresponding organoid models were profiled for protein expression of relevant endometrial cancer markers: ER, PR, L1CAM, PTEN, ARID1A, and assigned to molecular subgroups according to the TCGA-like classification by evaluating IHC staining of P53 and MMR proteins (MSH6, MSH2, PMS2, and MLH1), and by Sanger sequencing of POLE41 (Fig. 2a).