Interleukin-1 receptor associated kinase-4 (IRAK-4) has been identified as a central signal transduction mediator of the Toll-like receptor (TLR) and Toll/interleukin-1 receptor (TIR) pathways in vertebrate innate immunity. An IRAK-4 homologue was cloned from the black tiger shrimp (Penaeus monodon) (PmIRAK-4) and it shares domains and structures with other IRAK-4s. It was found to be mainly expressed in the hemocytes and midgut but also to a lower extent in several other tissues in shrimp. The PmIRAK-4 responded to bacterial infection in the intestine by an enhancement of its expression level. These results indicate that PmIRAK-4 may play a role at least in the intestinal innate immunity of P. monodon. (C) 2011 Elsevier Inc. All rights reserved.