In addition, the interaction of PCD-1 with its ligand can also affect the cell cycle (by increasing the expression of p15 decreasing the transcription level of S-phase Kinase-associated Protein-2 (SKP2), precluding T-cells from entering the G1 phase, and eventually inhibiting cell proliferation), stimulating tumor-specific T-cell apoptosis and promote differentiation of T-helper (Th) cells into Tregs.4 The gene discussed is SKP2; the disease is neoplasm.