FABP9 is a fatty acid-binding protein with primary expression in testicular cells and involvement in lipid transport and metabolism. In the testis, FABP9 is expressed in Leydig cells 1 and serves as a major structural component of the sperm perforatorium and perinuclear theca, sharing developmental origins and protein constituents with the postacrosomal sheath 2. While FABP9 knockout mice develop sperm head abnormalities such as ventral spur reduction, these mice remain viable and fertile, indicating FABP9 plays only a minor role in spermatogenesis 3. Human FABP9 mutations were not detected in infertile men with sperm morphological defects 4, suggesting limited pathogenic impact on human male fertility. Clinically, FABP9 demonstrates significant prognostic relevance in cancer. In prostate cancer, FABP9 is highly overexpressed in malignant cell lines and tissue samples, with expression correlating significantly with reduced patient survival, elevated Gleason scores, and androgen receptor index; suppression of FABP9 inhibited cancer cell invasion 5. FABP9 also serves as a risk gene in esophageal squamous cell carcinoma prognostication 6 and as part of an immune-related prognostic signature in breast cancer 7. These findings establish FABP9 as a valuable biomarker for predicting cancer outcomes, particularly in prostate malignancies.