Dekkers et al. successfully modelled ER-positive breast cancer tumours responsive to chemotherapy and immunotherapy by inducing targeted KOs of TP53, PTEN, Retinoblastoma gene (RB1), and Neurofibromatosis type 1 (NF1) with specific CRISPR/Cas9 sgRNAs in organoids. The gene discussed is TP53; the disease is neoplasm.