<p>The lipocalins are a diverse, interesting, yet poorly understood family of proteins composed, in the main, of extracellular ligand-binding proteinsdisplaying high specificity for small hydrophobic molecules [<cite idref="PUB00005094"/>]. Functionsof these proteins include transport of nutrients, control of cell regulation, pheromone transport, cryptic colouration, and the enzymatic synthesisof prostaglandins.</p><p>The crystal structures of several lipocalins have been solved and show a novel 8-stranded anti-parallel beta-barrel fold well conserved within thefamily. Sequence similarity within the family is at a much lower level andwould seem to be restricted to conserved disulphides and 3 motifs, whichform a juxtaposed cluster that may act as a common cell surface receptorsite [<cite idref="PUB00000205"/>, <cite idref="PUB00005013"/>]. By contrast, at the more variable end of the fold are found an internal ligand binding site and a putative surface for the formation of macromolecular complexes [<cite idref="PUB00003448"/>]. The anti-parallel beta-barrel fold is alsoexploited by the fatty acid-binding proteins, which function similarly bybinding small hydrophobic molecules. Similarity at the sequence level,however, is less obvious, being confined to a single short N-terminal motif.</p><p>This entry represents ApoD type Lipocalin, including Retinol-binding protein 4 as well as other retinol-binding proteins. Apolipoprotein D (ApoD) occurs in the macromolecular complex and is involved in lecithin-transport and binding of bilin. It appears to be able to transport a variety of ligands in a number of different contexts.</p> Lipocalin, ApoD type