Although cultured CD4 + and CD8 + T-cells can engage both the perforin and the Fas/FasL pathways, in vivo experiments suggest that MHC class I-dependent killing is dominated by the perforin pathway, whereas MHC class II-dependent elimination is almost entirely mediated by Fas/FasL interactions (Graubert et al., 1997; Schulz et al., 1995) .