The group-specific component (Gc) is the major vitamin D-binding protein in plasma. The gene encoding Gc is linked, on human chromosome 4, to the albumin and alpha-fetoprotein genes. These two genes previously were shown to have evolved from a smaller ancestral gene by intragenic triplication. Recombinant plasmids containing human cDNA encoding Gc have been isolated by screening an adult human liver library with a mixed oligonucleotide probe. Characterization of Gc cDNA has provided the complete amino acid sequence of the protein and revealed strong sequence homology with albumin and alpha-fetoprotein. Of particular interest is a conserved pattern of disulfide bridges that form the triple-domain structures in albumin, alpha-fetoprotein, and Gc. Gc cDNA was used as a probe in Southern blot analysis of somatic-cell hybrids to confirm that the Gc locus is on chromosome 4.