Here we describe the highly conserved gene, defender against apoptotic death (DAD1) identified from an EST library of the black tiger shrimp Penaeus monodon. The full-length cDNA of DAD1 of P. monodon comprised 638bp with an ORF of 345bp corresponding to 114 deduced amino acids. The deduced amino acid sequence was compared to known DAD1 sequences in the GenBank and in other databases. Phylogenetic analysis revealed that P. monodon DAD1 clustered with DAD1 from other invertebrates. Real-time RT-PCR with RNA extracts from normal P. monodon revealed DAD1 expression in several tissues including those of digestive and defense organs such as the hepatopancreas and hemocytes, respectively. If death from YHV infection was related to increased levels of apoptosis, we reasoned that the level of DAD1 should decrease as YHV infections progressed, especially in hemocytes (HC), one of its main targets. Real-time RT-PCR with RNA extracts from HC of P. monodon challenged with YHV revealed that the transcriptional level of DAD1 declined dramatically (approximately 50%) after YHV challenge. Although this suggests that DAD1 plays a role in mortality caused by YHV, control of apoptosis is complex and involves the interaction of many proteins, few of which have been characterized for shrimp. Thus, firm conclusions regarding the role of DAD1 must await the description and characterization of other proteins.