Conditional deletion of Fas in T cells, B cells and DC subsets has shown that the accumulation of CD4−CD8−B220+ T cells and DCs is dependent on the cell-intrinsic lack of Fas in T cells and DC subsets, respectively10, whereas accumulation of effector T cells is a consequence of autoimmunity and does not depend on lack of Fas on T cells8, 10. The gene discussed is FAS; the disease is Autoimmunity.