Activity of 2B4 mutants. YTS cells expressing different 2B4 mutants described in Figure 1 were tested in a redirected 51Cr-release assay against P815 target cells in the presence of an anti-HA (A,C) or an anti-NTB-A (B) antibody at different effector-to-target (E/T) ratios. Specific lysis in the presence of an isotype control antibody was less than 2% for all effector cells and E/T ratios. The Y3F mutant also showed a similar cytotoxic capacity when compared to the wild-type (WT) receptor (data not shown).