MDSCs are known to be a heterogeneous cell population based on the expression of CD11b, Ly6C, and Ly6G: Monocytic MDSCs (Mo-MDSCs; CD11b+Ly6Chi Ly6G−) and granulocytic MDSCs (G-MDSCs; CD11b+Ly6Clow Ly6G+).18,23,24 It has been shown that Mo-MDSCs from metastatic breast cancer patients are immunosuppressive and the proportion of Mo-MDSCs correlates with breast cancer progression and metastasis in human breast cancer patients.25 In the present study, we found that ibrutinib treatment significantly reduced the Mo-MDSCs in the spleens and tumours (Fig. 3d, e). The gene discussed is ITGAM; the disease is breast cancer.