ARID1A and neoplasm: In contrast to the observations that Arid1a loss promoted PCa progression in immunocompetent mice, we observed that Arid1a KO exhibited the inhibitory effects when tumor cells were engrafted in Rag1 null mice (Fig. 2e), suggesting that tumor or innate immune cell mediated mechanisms might account for a negative role of ARID1A loss on tumor growth.