Computer graphics image of human prealbumin (also called transthyretin), a plasma protein which acts as a storage & carrier molecule for the thyroid hormone thyroxine (T4). It is also independently involved in the transport of Vitamin A due to its ability to bind retinol-binding protein. Transthyretin consists of a functional tetramer composed of four identical subunits, each of molecular weight 14, 000 and formed of a chain of 127 amino-acid residues. Here, the alpha carbon backbone of each monomer is shown in orange, purple, blue & green. The T4 binding channel runs through the centre of this image.