Conversely, increased p21 expression (Supplementary Fig. 5d, g, i) induced by treating MDMs with phorbol myristate acetate (PMA)29, the histone deacetylase (HDAC) inhibitor MS27530 or immobilized immunoglobulins (intravenous immunoglobulins, IVIg), which bind macrophage Fcγ receptors and upregulate p21 expression31, significantly enhanced the phagocytosis of Jurkat cells and MOLT4 cells (Supplementary Fig. 5e, f, h, j), confirming that p21 regulates the MDM-mediated phagocytosis of leukemia cells. Here, HDAC9 is linked to leukemia.