we performed proteome analysis of Th9 cells to understand the involvement of proteins that might be crucial for the anti-tumor functions of Th9 cells. Here we show for the first time a comprehensive proteomic analysis of murine Th0 and Th9 cells, which identified 1451 total proteins among which 1367 proteins were common. Further analysis revealed that 118 proteins were upregulated while 81 proteins were downregulated in Th9 cells. Pathway analysis suggested an abundance of phosphoproteins in the proteome of Th9 cells. Among upregulated phosphoproteins which showed to be involved in immune 34 system, Ppp2ca (catalytic subunit of protein phosphatase, PP2A) was found to be highly expressed in Th9 cells. Although the role of PP2A has been shown to regulate the differentiation and functions of Th1,Th2, Th17 and Tregs, its role in differentiation and functions of Th9 cells is not identified yet. Our results show for the first time that PP2A is required for the differentiation and anti-tumor functions of Th9 cells. PP2A inhibition leads to the suppression of IL-9 induction and the expression of key transcription factors of Th9 cells.