KIF9 (kinesin family member 9) is a microtubule motor protein essential for male fertility and multiple cellular processes. Primary Function: KIF9 mediates anterograde transport of intracellular cargo along microtubules and regulates flagellar motility in spermatozoa 1. Unlike its family member Kif6, KIF9 functions as a central apparatus regulator rather than a traditional motor, oscillating regionally on the ciliary central apparatus 1. Mechanism: KIF9 localizes to the central microtubule pair in sperm flagella and interacts with the microtubule component HYDIN 2. In non-reproductive contexts, KIF9 localizes to centriolar satellites and regulates their pericentrosomal positioning, controlling centrosome maturation and centriolar protein trafficking 34. KIF9 mediates lysosomal transport through kinesin light chain 1 (KLC1), promoting macroautophagy 5. Disease Relevance: KIF9 loss-of-function variants cause asthenozoospermia (reduced sperm motility) and male infertility 21. KIF9 deficiency impairs centrosome maturation, causing chromosome 3 defects 3. Reduced KIF9 expression correlates with Alzheimer's disease pathology; KIF9 upregulation enhances macroautophagy and ameliorates cognitive dysfunction by reducing amyloid-beta accumulation 5. Clinical Significance: In infertile males with KIF9 mutations, in vitro fertilization successfully overcomes fertility issues 2. KIF9-AS1, a long non-coding RNA, serves as a biomarker for inflammatory bowel disease and hepatocellular carcinoma 67.