Nature Immunology 5, 524–530 (2004).

In the legend to Figure 4, the last sentence should be replaced with the following: “Curves in b and d represent the average of at least eight CTL-APC conjugates each.” The authors also regret having omitted reference to M. Faroudi et al. Lytic versus stimulatory synapse in cytotoxic T lymphocyte/target cell interaction: manifestation of a dual activation threshold. Proc. Natl. Acad. Sci. USA 100, 14145–14150 (2003), which showed that with a human cytotoxic T lymphocyte line, lower concentrations of the antigenic peptide than were needed for mature synapse formation could nonetheless induce cytotoxicity. This should be cited on page 528, column 2, at the end of the first paragraph.