For example, binding of PD-1 on T cells to PD-L1 expressed on tumor cells or antigen presenting cells (APCs) would first recruit protein tyrosine phosphatase-2 (SHP2) to dissociate proximal signaling molecules downstream of the T cell antigen receptor (TCR) and CD28 (Nagai, 2020a). This evidence concerns the gene CD28 and neoplasm.