The recently emerged Middle East respiratory syndrome coronavirus (MERS-CoV), a betacoronavirus, is associated with severe pneumonia and renal failure. The environmental origin of MERS-CoV is as yet unknown; however, its genome sequence is closely related to those of two bat coronaviruses, named Bt. CoV-HKU4 and Bt. CoV-HKU5, which were derived from Chinese bat samples. A hallmark of highly pathogenic respiratory viruses is their ability to evade the innate immune response of the host. CoV accessory proteins, for example those from severe acute respiratory syndrome CoV (SARS-CoV), have been shown to block innate antiviral signalling pathways. MERS-CoV, similar to SARS-CoV, has been shown to inhibit type I IFN induction in a variety of cell types in vitro. We therefore hypothesized that MERS-CoV and the phylogenetically related Bt. CoV-HKU4 and Bt. CoV-HKU5 may encode proteins with similar capabilities. In this study, we have demonstrated that the ORF4b-encoded accessory protein (p4b) of MERS-CoV, Bt. CoV-HKU4 and Bt. CoV-HKU5 may indeed facilitate innate immune evasion by inhibiting the type I IFN and NF-ฮบB signalling pathways. We also analysed the subcellular localization of p4b from MERS-CoV, Bt. CoV-HKU4 and Bt. CoV-HKU5 and demonstrated that all are localized to the nucleus. ยฉ 2014 SGM.