Naturally occurring retinoid originally discovered in livers of fresh-water fish. Constitutes 20-25% of total retinoid content in human epidermis; levels are markedly elevated in patients with hyperproliferative dermatoses such as psoriasis. Discovery in freshwater fish: E. Lederer et al., Nature 140, 233 (1937); J. R. Edisbury et al., ibid. 234. Isoln from pike liver oils: E. M. Shantz, Science 108, 417 (1948). Structure and synthesis: K. R. Farrar et al., J. Chem. Soc. 1952, 2657. Synthesis and characteristics of stereoisomers: U. Schwieter et al., Helv. Chim. Acta 45, 517, 528, 541, 548 (1962). HPLC determn in fish oils: B. Stancher, F. Zonta, J. Chromatogr. 312, 423 (1984). Identification in human skin: A. Vahlquist, Experientia 36, 317 (1980). Comparative levels in psoriasis patients: O. Rollman, A. Vahlquist, Arch. Dermatol. Res. 278, 17 (1985). Biosynthesis from retinol, q.v., in cultured keratinocytes: O. Rollman et al., Biochem. J. 293, 675 (1993).