In a study using an HIV-based pseudovirus system and cell  lines expressing human, civet and horseshoe bat ACE2 molecules, the bat SL-CoV Rp3 S protein demonstrated its inability to use ACE2 as cell receptor. However, the chimeric Rp3 S protein carrying the receptor-binding domain of SARS-CoV S protein was conferred the capability of cell entry via human ACE2.  These results suggested that bat SL-CoVs such as Rp3 were unlikely to cause human infection. Therefore, they may not be considered as the direct progenitor of SARS-CoV. Besides, the theory of bat origin of SARS-CoV lacked a powerful support due to the failure of  direct isolation of SL-CoV from bats, despite numerous trials.