In addition, recombinant RBDs (residues 318–510 or 318–536) stably or transiently expressed in Chinese hamster ovary (CHO) cells bound strongly to RBD-specific monoclonal antibodies (mAbs), elicited high-titer anti-SARS-CoV neutralizing antibodies, and protected most, or all, of the SARS-CoV-challenged mice, with undetectable viral RNA and undetectable or significantly reduced viral load (Du et al., 2009c, 2010).