Fatty acid binding protein 9 (FABP9) belongs to the fatty acid-binding proteins (FABPs) which are a family of small, extremely conserved, cytoplasmic proteins to bind long-chain fatty acids and other hydrophobic ligands. FABP9 is found in midpachytene spermatocytes and round spermatids, and comprises part of the perinuclear theca. FABP9 probably links intracellular membranes and signals abnormal sperm formation during spermatogenesis.