Prealbumin, also designated transthyretin, is a major thyroid-hormone binding protein involved in transporting thyroxine from the bloodstream to the brain. Prealbumin is located in the cytoplasm and in the vesicles of developing rat brain cells, and is thought to be transported there from the cerebrospinal fluid via endocytosis. Sequence variants of this protein have been identified in amyloid fibrils from patients with familial amyloidotic polyneuropathy (FAP), the most common form of hereditary systemic amyloidosis. Although the biologically active form of Prealbumin is a tetramer, the amyloidogenic intermed-iate is thought to be a monomeric species. Prealbumin also binds to the retinol carrier protein RBP (retinol-binding protein). The gene encoding Prealbumin maps to human chromosome 18q12.1.