CD33 and neoplasm: In addition to the three main populations, various new definitions of MDSC have been identified in different environments, such as CXCR1+CD15−CD14+HLA-DR−/low [13] PD-L1+ CD11b+CD33+HLA-DR− [14] MDSC in tumor microenvironments secreted protein acidic and rich in cysteine (SPARC)-positive MDSC in inflammatory state [15], while it remains unknown whether these MDSCs are truly distinct from classical G-MDSCs, M-MDSCs, or eMDSCs.