PIWIL4: from biological characteristics to disease pathogenesis and clinical applications.

PIWIL4 is a core component of the PIWI/piRNA pathway. Its roles in germ cell development, transposon silencing, and genome stability maintenance are well established. Recent studies have shown that PIWIL4 is abnormally expressed in many cancers. There, it helps regulate key malignant processes-including cell cycle control, apoptosis, epithelial-mesenchymal transition, stemness maintenance, and metabolic reprogramming. However, its effects vary widely, it can promote or suppress tumours depending on the cancer type, and even across subtypes of the same cancer. This striking functional duality is highly context-dependent. This review systematically covers the molecular structure, classical functions, and complex regulatory networks of PIWIL4 in both cancerous and non-cancerous conditions. We focus on the molecular underpinnings of this duality-specifically, piRNA-guided targeting, differences in protein interaction networks, and the influence of cellular context. We also assess its promise and hurdles as a diagnostic biomarker and therapeutic target. A deeper understanding of these context-dependent mechanisms should inform the design of targeted interventions in precision oncology.