A number of serum transport proteins are known to be evolutionarily related, including albumin, alpha-fetoprotein, vitamin D-binding protein and afamin [<cite idref="PUB00003407"/>, <cite idref="PUB00000582"/>, <cite idref="PUB00002857"/>]. Albumin is the main protein of plasma; it binds water, cations (such as Ca²⁺, Na⁺ and K⁺), fatty acids, hormones, bilirubin and drugs - its main function is to regulate the colloidal osmotic pressure of blood. Alphafeto- protein (alpha-fetoglobulin) is a foetal plasma protein that binds various cations, fatty acids and bilirubin. Vitamin D-binding protein binds to vitamin D and its metabolites, as well as to fatty acids. The biological role of afamin (alpha-albumin) has not yet been characterised. The 3D structure of human serum albumin has been determined by X-ray crystallography to a resolution of 2.8A [<cite idref="PUB00004128"/>]. It comprises three homologous domains that assemble to form a heart-shaped molecule [<cite idref="PUB00004128"/>]. Each domain is a product of two subdomains that possess common structural motifs [<cite idref="PUB00004128"/>]. The principal regions of ligand binding to human serum albumin are located in hydrophobic cavities in subdomains IIA and IIIA, which exhibit similar chemistry. Structurally, the serum albumins are similar, each domain containing five or six internal disulphide bonds, as shown schematically below:<pre> +---+ +----+ +-----+ | | | | | | xxCxxxxxxxxxxxxxxxxCCxxCxxxxCxxxxxCCxxxCxxxxxxxxxCxxxxxxxxxxxxxxCCxxxxCxxxx | | | | | | +-----------------+ +-----+ +---------------+</pre><p>This entry represents a subgroup of serum albumin.</p> Serum albumin, subgroup