AK9 is a broad-specificity nucleoside phosphate kinase that catalyzes nucleoside monophosphate phosphorylation, preferentially converting AMP to ADP using ATP as phosphate donor 1. AK9 also exhibits diphosphate kinase activity, producing various NTPs from their diphosphate precursors with substrate preference CDP > UDP > ADP > TDP 1. Localized to the nucleus, cytoplasm, and nucleoplasm, AK9 maintains cellular nucleotide homeostasis and energy metabolism 2. In sperm, AK9 functions as a critical component of the adenylate kinase phosphate energy shuttle, localizing to the flagellar axoneme radial spoke where it cooperatively regulates ATP transfer with AK8 3. Bi-allelic AK9 mutations cause asthenozoospermia (reduced sperm motility) through destabilization of sperm nucleotide homeostasis and inhibition of glycolysis, though intracytoplasmic sperm injection effectively rescues fertility 2. AK9 mutations are implicated in neuromuscular and neurological disorders: digenic AK9/RAPSN mutations cause congenital myasthenic syndrome through impaired N-glycosylation dependent on nucleotide availability 4, and heterozygous AK9 mutations contribute to idiopathic normal pressure hydrocephalus by impairing ependymal ciliary motility 5. Additionally, AK9 functions as a tumor suppressor in lung cancer, with rs1321328 reducing AK9 expression through altered m6A methylation 6.