CD98 (4F2) belongs to a group of approximately two hundred proteins at the surface of leukocytes that are thought to be important for their function in the immune response, by specifying their migration, cell interactions and the signal transduction mechanisms that lead to lymphocyte activation. Following the discovery of CD98 (4F2) as a surface antigen in lymphocytes (Haynes et al., 1981), the antigen was found to be present in all established human tissue culture cell lines tested and in most malignant human cells (Hemler & Strominger, 1982; Quackenbush et al., 1987). Although it was linked to cell proliferation and growth (Yagita et al., 1986, Gottesdiener et al., 1988) it essentially remained a protein in search of a function. Only recently, have reports linked CD98 (4F2) to a number of specific cellular processes such as adhesion, fusion and amino acid transport across the cell membrane in different cell types. How these relate to its role in cell activation and proliferation, has not yet been established.

The complete molecular structure of CD98 (4F2) has been resolved during the last year. Early biochemical studies showed that the 4F2 cell surface antigen (which was later designated CD98; Barclay et al., 1997) was a 120-kDa disulfide-linked heterodimer composed of two subunits, a glycosylated 80 kDa heavy chain and a 40