AKAP4 (A-kinase anchoring protein 4) is a major structural component of the sperm fibrous sheath that plays a critical role in sperm motility and flagellar assembly 12. The protein exists as a precursor (pro-AKAP4) that localizes to the fibrous sheath of the sperm flagellum and serves as a reservoir of mature AKAP4 2. AKAP4 functions by anchoring protein kinase A (PKA) to the fibrous sheath structure and acts as a molecular switch between cAMP/PKA and PKC/ERK1/2 signaling pathways, regulating sperm capacitation and acrosome reaction 3. The protein is phosphorylated by STK33, which is essential for proper fibrous sheath assembly during spermiogenesis 1. AKAP4 mutations are associated with male infertility, particularly asthenozoospermia, where missense mutations can cause protein mislocalization, increased oxidative stress, and impaired PKA/PI3K signaling 4. Clinically, AKAP4 serves as a biomarker for identifying rare spermatozoa in non-obstructive azoospermia patients, potentially enabling non-invasive diagnostics prior to surgical sperm retrieval 5. Additionally, AKAP4 is aberrantly expressed in various cancers including esophageal and colorectal cancer, where it promotes tumor growth and metastasis 67.