PIWIL1 and plasma cell myeloma: Gastric cancers overexpress piR-651, promoting G2/M arrest evasion; lung cancers feature PMLCPIR enhancing ITGB1-PI3K-AKT signaling; multiple myeloma leverages piR-823 for proliferation; and hepatocellular carcinoma shows PIWIL1 upregulation tied to stemness.