Mammalian amino acid transport systems consist of a large variety of transporters. The amino acid transporter, cystine/glutamic acid transporter xCT, for system xc has been proposed to be responsible for the cystine transport through the plasma membrane. System xc mediates an amino acid exchange and prefers cystine and glutamate as its substrates. The transporter designated as xCT requires 4F2 heavy chain (4F2hc:CD98) for its functional expression, and belongs to the family of amino acid transporters that associates with the type II membrane glycoproteins such as 4F2hc. In brain, it has been proposed that system xc is up-regulated in glial cells upon oxidative stress and plays an essential roles to protect neurons against oxidative stress. Recent studies have implicated xCT as a gateway that Kaposi's sarcoma herpesvirus (KSHV) uses to enter cells.