orf390 (WSSV449) is a novel apoptosis inhibitor gene in the genome of the White Spot Syndrome Virus (WSSV). In the present study, we focus on the function of orf390 gene. Stable expression of orf390 prevented SF9 insect cells from undergoing actinomycin D-induced apoptosis. ORF390 also rescued the replication of a p35-deficient-mutant (AcMNPV Delta p35k/pol+) in SF9 cells. In addition, ORF390 inhibits the activities of caspase-3 and -9 in vivo and in vitro. Here we demonstrate that the anti-apoptotic activity of ORF390 is dependent on two putative caspase-9 cleavage sites (VETD2334 down arrow G and LEHD303,down arrow G) and one caspase-3 cleavage site (DEVD272 down arrow G). Our results support the conclusion that these three sites play a key role in the suppression of apoptosis mediated by ORF390. These data further suggest that orf390 encodes a novel anti-apoptotic protein involved in cell survival and apoptosis regulation. (C) 2010 Published by Elsevier B.V.