Susceptibility to enhanced apoptosis in vitro and to anti–Fas-triggered apoptosis at various time points after BMT. Percentages of cells with low ▵Ψm were determined by cytofluorometry in gated CD4⁺ and CD8⁺T-cell subsets. Rates of apoptosis were much higher (P > .0001) on days 45 to 90 and 180 to 365, but not on day 730 than those of control cells. The P values (asterisks) show the levels of anti–Fas-triggered apoptosis relative to spontaneous background apoptosis. Histograms are means ± SD for individual determinations.