PDCD1 and cancer: In T cells, interaction of PD-1 with PD-L1/PD-L2 inhibits TCR signaling via SHP-2 tyrosine phosphatase.14 SHP-2 expression is reported in a number of cancer cell types, including melanoma,128 breast cancer129 and glioblastoma.130 In cancer cells, SHP-2 may activate mTOR signaling130; hence, the effect of PD-1 signaling on mTOR activation might be tissue specific.