RAG1 and neoplasm: Strikingly, we found that in immunocompetent mice, 9609‐MEK1‐hi tumours grew significantly slower than 9609‐MEK1‐low tumours in WT (p < 0.01 Figure 2b) but not RAG1−/− animals (Figure 2c), suggesting that the reduced growth of MEK1‐hi tumours requires the adaptive immune system.