In addition, we developed a novel transgenic mouse model that supports MERS-CoV infection by replacing the mouse DPP4 coding sequence with that encoding human DPP4. Using the VelociMouse method, F0 humanized mice fully derived from ES cells were available for analysis within 4 mo following gene targeting. We used this model to demonstrate that the identified antibodies can prophylactically protect mice from infection as well as ameliorate a previously established infection in a postinoculation treatment paradigm. As such, these two antibodies are promising candidates for prophylaxis and treatment of MERS-CoV infections. Two other mouse models have been developed for MERS-CoV. The first model used an adenovirus delivery of huDPP4 intranasally into a mouse, resulting in many cells expressing the huDPP4 receptor and efficient MERS-CoV replication. However, a concern with this model is that cells not natively expressing DPP4 will be infected and the role of a broader infection of cell types may alter pathogenesis (21). More recently another model has been published where huDPP4 is under the control of the constitutively expressing chicken β-actin promoter, driving expression of huDPP4. In this model, all cells of the mouse express huDPP4, a nonphysiological expression pattern, and these mice develop extensive brain infection of MERS-CoV and rapidly succumb to infection (23). Our model replaces the mDPP4 ORF with the huDPP4; thus, the newly knocked-in huDPP4 is under the control of the endogenous mDPP4 promoter, ensuring correct physiological expression of the knocked-in gene, providing a more physiological model of human disease. Further characterization of this model will allow us to understand the host response to MERS-CoV. Additionally, the huDPP4 mouse model can also be used for the efficient testing of drugs and vaccines in a small animal model, allowing for rapid identification of additional therapeutics to identify leads for testing in nonhuman primates before they are proposed for use in humans.