AK8 (adenylate kinase 8) is a nucleoside monophosphate kinase that catalyzes the reversible transfer of phosphate groups between nucleoside triphosphates and monophosphates, with highest activity toward AMP and weaker activity toward dAMP, CMP, and dCMP 1. The enzyme contains two functional domains within its polypeptide chain, both showing similar enzymatic activity, and demonstrates cytosolic localization 1. AK8 plays a critical role in ciliary and flagellar motility by functioning as part of the radial spoke structure in motile cilia and sperm flagella 23. The protein requires EFCAB10 for proper anchoring to the radial spoke, as AK8 is completely absent from cilia in Efcab10-deficient mice 2. In sperm, AK8 cooperatively regulates ATP transfer within the flagellar axoneme alongside AK9, concentrating at sites of high energy consumption 3. AK8 also participates in an adenine nucleotide homeostasis module with CFAP45, supporting flagellar beating through AMP/ADP modulation 4. Loss of AK8 function leads to primary ciliary dyskinesia-related phenotypes and male infertility due to impaired sperm motility 23. The protein serves as a component of radial spoke 1 (RS1) structure, specifically forming part of the RS1 stalk 5.