MAPK9 and bacterial infectious disease: Genes that were up-regulated in older but not in younger flies include PGRP-LB, which inhibits activation of the Imd pathway in response to bacterial infection by cleaving DAP-type peptidoglycans, and the cytokine Unpaired 2 (Upd2), which is a transcriptional target of the Jun kinase (JNK) pathway and activates the JAK-STAT pathway (Zaidman-Rémy et al. 2006; Rajan and Perrimon 2012).