Our co-culture data obtained from IDO deficient MDSCs and B cells demonstrate for the first time that IDO associated MDSCs induces Breg ex-vivo. In addition, reduced Breg frequency in a syngeneic tumor model with conditional deletion of IDO in myeloid cells (IDO f MCre) further support the positive regulation by MDSC- associated IDO in Breg differentiation in-vivo. This evidence concerns the gene IDO1 and neoplasm.