Animal models have greatly contributed to the understanding of different aspects of human biology, as well as a variety of human-related pathogens and diseases. In order to study them, humanized mice susceptible to pathogens that replicate in human immune cells have been developed (e.g., humanized Rag2-/-γc-/- mice). These animals are engrafted with human hematopoietic stem cells (HSCs), resulting in the de novo development and maturation of the major functional components of the human adaptive immune system and the production of a variety of human cell types. Primary and secondary lymphoid organs in the mouse are populated with human cells, and animals have long term engraftment. These features make humanized mice an excellent in vivo model to study pathogenesis of human-specific viruses in the context of a human antiviral immune response. In addition, humanized mice have been shown to be useful preclinical models for the development and validation of antiviral therapeutics. In the present study, we aimed to successfully re-establish the humanized Rag2-/-γc-/- mouse model using cord blood-derived HSCs in our laboratory. We have shown that these mice sustain long term engraftment and systemic expansion of human cells, including the major targets of Kaposi's sarcoma Herpesvirus (KSHV) and Human immunodeficiency virus type 1 (HIV-1), in peripheral blood and different lymphoid organs. Further, we have begun to evaluate the susceptibility of the humanized Rag2-/-γc-/- mouse model to infection with KSHV. We demonstrate that human lymphocytes differentiated in reconstituted Rag2-/-γc-/- mice are permissive to KSHV infection ex vivo. This finding was corroborated by detection of KSHV mRNA expression in the spleen of a humanized mouse at 6 months post infection. In a different study, we tested the in vivo antiviral efficacy of a novel HIV-1 fusion inhibitor (PIE-12-trimer) in humanized Rag2-/-γc-/- mice. We have determined the half life of PIE-12-trimer in mouse plasma. Furthermore, the administration of PIE-12-trimer to HIV-1 infected humanized Rag2-/-γc-/- mice prevents depletion of CD4+ T cells in blood, thus it may be useful to prevent AIDS in human patients.