STAT3 and neoplasm: Two theories have been proposed for expansion, activation and migration of MDSCs in tumor model: (i) “One signal hypothesis” states that one signal is enough for the differentiation of hematopoietic progenitor cells into MDSCs (ii) “Two signal hypothesis” states that the differentiation of HSC to MDSCs occurs in two steps: (i) HSC-IMC transition via STAT-3 signal on activation with cytokines like G-CSF,GM-CSF,IL-6,PGE2 and (ii) IMC-MDSCs transition mediated by mainly proinflammatory cytokines LPS, S100 proteins, IL-1 ß etc. and activating NFκ-B, PI3K, and STATs mediated signaling.