IGFBP4 and neoplasm: Third, among the expression upregulated human apoptotic-related markers, as DR6, IGFBP-4, p21, p53, and TRAILR-3 after PSMC2 knockdown, p21, a CDK inhibitor, was the most significant upregulated, and was reported to participate in cell cycle progression and could play a crucial role in tumor development by the p53 pathway [15–18].