Notably, treatment with PD98059 reduced growth of both MEK1‐hi and MEK‐low tumours in vitro (Figure S7B), suggesting that the increased growth effect of MEK1 inhibition in mice acted on a host cell rather than the tumour cell and must have outweighed the anti‐proliferative effects of MEK1 inhibition on the tumour cell itself. Here, MAP2K1 is linked to neoplasm.