ENO4 (enolase 4) is a sperm-specific glycolytic enzyme that catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate, a critical step in glycolytic ATP production. Located in the sperm principal piece, ENO4 is essential for sperm flagellar organization and motility 1. The enzyme functions within the axonemal structure of the sperm flagellum, where it supports the energy demands required for sperm cell movement 2. Clinically, ENO4 dysfunction directly causes male infertility. A homozygous variant (c.293A>G, p.Lys98Arg) in ENO4 segregated with asthenozoospermia and abnormal sperm morphology in an affected family, representing the first documented human case of ENO4-associated infertility 1. Environmental fluoride exposure downregulates ENO4 expression in testicular tissue, disrupting sperm flagellar ultrastructure and reducing sperm quality 2. Additionally, ENO4 expression is dysregulated in autoimmune thyroid disease, where reduced ENO4 levels correlate with impaired ciliogenesis, suggesting broader roles in organellar function beyond sperm 3. A breast cancer susceptibility locus (rs1626678) near ENO4 at chromosome 10.3 shows association with increased breast cancer risk 4, though the mechanistic relationship remains unclear.