We found that after 72 h of culture in medium alone, a lower proportion of CLL cells from the high CTLA-4 expressors became apoptotic compared to those from the low CTLA-4 expressors, which corresponded with the significant down-regulation of surface CTLA-4 expression only in the high CTLA-4 expressors. The gene discussed is CTLA4; the disease is B-cell chronic lymphocytic leukemia.