PD-1 (programmed cell death protein 1, expressed on activated T cells) was not expressed in either the tumor cells or stromal cells of PPCP and APCP (Fig. 4C, D), although PD-1 was detected in the stromal cells of both PPCP and APCP through immunofluorescence staining (Fig. 4I). Here, PDCD1 is linked to neoplasm.