CD4 and HIV-1 infection: As shown in Fig. 2, the homologies between tree shrew and human CD4, CCR5, and CXCR4 were 61.0%, 78.1%, and 97.7%, respectively (aligned by ClustalW; https://www.genome.jp/tools-bin/clustalw), suggesting that CD4 and CCR5 may be limiting factors for HIV-1 infection in tree shrew cells.