FABP9, also known as fatty acid binding protein 9, is a 132 amino acid protein. It is a member of fatty acid-binding proteins (FABPs) which are a family of carrier proteins for fatty acids and other lipophilic substances such as eicosanoids and retinoids. These proteins are thought to facilitate the transfer of fatty acids between extra- and intracellular membranes. FABP9 is found in midpachytene spermatocytes and round spermatids, and constitutes part of the perinuclear theca. Functionally, FABP9 is likely to link intracellular membranes, and may signal abnormal sperm formation during spermatogenesis.