Many of the early reports detailing the appearance of CD4 CTL did not identify the mechanism of cell killing by clones generated in vitro although, by the 1990s, it was reported that the cell surface receptor:ligand pair, Fas:FasL, was a major mechanism of cell death induced by CD4 CTL during interaction with virally infected B cells (23) or in infection with LCMV (14). This evidence concerns the gene CD4 and infection.