SPAG9 (sperm-associated antigen 9) is a multifunctional scaffold protein with roles spanning reproductive biology, intracellular signaling, and cancer progression. As a member of the JNK-interacting protein (JIP) family, SPAG9 acts as a scaffold protein that mediates JNK signaling by aggregating MAPK cascade components 1. Beyond signaling, SPAG9 regulates lysosomal positioning by linking PIP4P1-positive lysosomes to the dynein-dynactin complex 2. In reproduction, SPAG9 localizes to the sperm acrosome and undergoes stage-specific relocation during the acrosome reaction, playing a key role in sperm-egg interaction and binding 3. The protein is highly conserved across primates, showing 84.9-96.8% sequence homology 4. Clinically, SPAG9 demonstrates contrasting roles depending on cancer type. It is overexpressed in prostate cancer, astrocytoma, and osteosarcoma, where it promotes cell proliferation, invasion, and angiogenesis through MMP-2/9 regulation and VEGF signaling 5678. Conversely, in clear-cell renal cell carcinoma, SPAG9 expression correlates with favorable prognosis, potentially through autophagy promotion and inflammatory suppression 9. Recently, biallelic SPAG9 mutations were identified in a novel human syndrome featuring coarse facial features, albinism, cataracts, and developmental delay, highlighting its multisystemic importance 10.