By combining this GEMM with organoid and patient-derived models, we showed that c-Src and forkhead box protein M1 (FOXM1), a key transcriptional regulator of the cell cycle (22), formed a positive feedback loop driving the cell-cycle progression that is required for efficient progression of luminal B–like (ER+Ki67hi) (23, 24) breast cancer beyond an early hyperplastic stage. Here, FOXM1 is linked to breast carcinoma.