<p>Calycins form a large protein superfamily that share similar beta-barrel structures. Calycins can be divided into families that include lipocalins, fatty acid binding proteins, triabin, and thrombin inhibitor [<cite idref="PUB00014136"/>]. Of these families, the lipocalin family (<db_xref db="INTERPRO" dbkey="IPR002345"/>) is the largest and functionally the most diverse. Lipocalins are extracellular proteins that share several common recognition properties such as ligand binding, receptor binding and the formation of complexes with other macromolecules. Lipocalins include the retinol binding protein, lipocalin allergen, aphrodisin (a sex hormone), alpha-2U-globulin, prostaglandin D synthase, beta-lactoglobulin, bilin-binding protein, and the nitrophorins [<cite idref="PUB00014141"/>, <cite idref="PUB00014138"/>, <cite idref="PUB00014139"/>, <cite idref="PUB00014140"/>].</p><p>This entry represents calycin as well as some other proteins that share a similar calycin beta-barrel structure, including two bacterial hypothetical proteins YodA from <taxon tax_id="562">Escherichia coli</taxon> and YwiB from <taxon tax_id="1423">Bacillus subtilis</taxon>. Part of the YodA hypothetical protein has a calycin-like structure [<cite idref="PUB00015733"/>].</p>