In the August 3 Science, Laxminarayana Devireddy and colleagues from the University of Massachusetts Medical School report the use of DNA microarrays to identify genes whose expresssion is induced during apoptosis (Science 2001, 293:829-834). They studied cell death of a mouse pro-B lymphocytic cell line upon withdrawl of interleukin-3. The gene that showed the largest induction (12.6-fold) was 24p3, which encodes a lipocalin. Lipocalins are small secreted proteins, and Devireddy et al. found that conditioned medium from dying lymphocytes induced cell death in a range of leukocytic cells. Recombinant 24p3 protein alone could also induce lymphocyte apoptosis. The cell death induced by 24p3 probably plays a role in immune-system homeostasis and in the regulation of the inflammatory response. The authors point out how important the expression profiling approach was in leading them to a unidentified killer.