CD33 and cancer: In humans, gMDSCs are phenotypically characterized as CD33+CD11b+HLA-DRlow/-CD15+CD14- and mMDSCs as CD33+CD11b+HLA-DRlow/-CD15-CD14+ [16]. All human MDSCs express Siglec-3/CD33 as a marker [16], but little is known about the expression of additional Siglec receptors on MDSCs in human cancer patients and their functional relevance [17].