Vitamin D-binding protein
Vitamin D-binding protein (DBP), also/originally known as gc-globulin (group-specific component), is a protein that in humans is encoded by the GC gene.

# Structure
Human GC is a glycosylated alpha-globulin, ~58 kDa in size. Its 458 amino acids are coded for by 1690 nucleotides on chromosome 4 (4q11q13). The primary structure contains 28 cysteine residues forming multiple disulfide bonds. GC contains 3 domains. Domain 1 is composed of 10 alpha helices, domain 2 of 9, and domain 3 of 4.

# Function
Vitamin D-binding protein belongs to the albumin gene family, together with human serum albumin and alpha-fetoprotein. It is a multifunctional protein found in plasma, ascitic fluid, cerebrospinal fluid and on the surface of many cell types.
It is able to bind the various forms of vitamin D including ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3), the 25-hydroxylated forms (calcifediol), and the active hormonal product, 1,25-dihydroxyvitamin D (calcitriol).  The major proportion of vitamin D in blood is bound to this protein.  It transports vitamin D metabolites between skin, liver and kidney, and then on to the various target tissues.
As Gc protein-derived macrophage activating factor it is a Macrophage Activating Factor (MAF) that has been tested for use as a cancer treatment that would activate macrophages against cancer cells.

## Interactive pathway map
Click on genes, proteins and metabolites below to link to respective articles. 
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# Production
It is synthesized by hepatic parenchymal cells and secreted into the  circulation.

# Genetic variation
Many genetic variants of the GC gene are known. They produce 6 main haplotypes and 3 main protein variants (Gc1S, Gc1F and Gc2). The genetic variations are associated with differences in circulating 25-hydroxyvitamin D levels. They have been proposed to account for some of the differences in vitamin D status in different ethnic groups, and have been found to correlate with the response to vitamin D supplementation.