GC (Group-Specific Component, also known as Vitamin D Binding Protein): GC, also known as Vitamin D Binding Protein, is a protein that binds and transports vitamin D and its metabolites in the bloodstream. It plays a key role in regulating calcium and phosphate balance, as well as modulating immune and inflammatory responses. Variations in GC can influence vitamin D levels and metabolism, affecting bone health and potentially altering the risk of autoimmune diseases.