Human cytomegalovirus (HCMV) is a major opportunistic pathogen and the cause of congenital disease and a range of debilitating diseases in immunocompromised patients. Extensive circumstantial evidence suggests that HCMV is harbored in peripheral white blood cells (as at least one site of latency). The animal model for the study of HCMV latency uses the severe combined immunodeficient mouse (C.B-17 scid/scid) transplanted with human implants (SCID-hu mouse) of bone with bone marrow (BM). The human hematopoietic cell population(s) harboring the HCMV genome in these animals and in BM explants will be identified and the results compared to normal adult seropositive individuals. Cell types will be identified by cell surface marker and PCR analysis will be used to detect viral DNA. The level of viral HCMV gene expression that occurs in cells carrying the HCMV genome will be compared to the pattern of viral transcription in naturally infected humans. This approach will use a bulk RT-PCR approach followed by isolation of products with homology to any portion of the HCMV genome. Finally, the state of cellular differentiation of cells harboring HCMV will be evaluated as a means of reactivating latent HCMV from infected SCID-hu mice or from normal seropositive individuals. Taken together, the proposed research will undertake studies that seek to fully define the biological relevance of various cell types in persistence of HCMV and to gain a deeper understanding of the function of viral genes in latency.