Herpesvirus gene expression is divided into immediate-early (IE) or ␣ genes, early (E) or  genes, and late (L) or ␥ genes on the basis of temporal expression and dependency on other gene products. By using real-time PCR, we have investigated the expression of 35 human herpesvirus 6B (HHV-6B) genes in T cells infected by strain PL-1. Kinetic analysis and dependency on de novo protein synthesis and viral DNA polymerase activity suggest that the HHV-6B genes segregate into six separate kinetic groups. The genes expressed early (groups I and II) and late (groups V and VI) corresponded well with IE and L genes, whereas the intermediate groups III and IV contained E and L genes. Although HHV-6B has characteristics similar to those of other roseoloviruses in its overall gene regulation, we detected three B-variant-specific IE genes. Moreover, genes that were independent of de novo protein synthesis clustered in an area of the viral genome that has the lowest identity to the HHV-6A variant. The organization of IE genes in an area of the genome that differs from that of HHV-6A underscores the distinct differences between HHV-6B and HHV-6A and may provide a basis for further molecular and immunological analyses to elucidate their different biological behaviors.In the mid-and late 1980s human herpesvirus 6A (HHV-6A) and HHV-6B were discovered and subsequently named as variants of 20,32,34). Although they share approximately 90% nucleotide sequence identity (13, 18), the accumulating biological, genetic, and epidemiological data suggest that these viruses have independent biological functions (13,18,34,37,38). Whereas little is known about the disease potential of HHV-6A, HHV-6B is the causative agent of the childhood disease exanthem subitum, characterized by a few days of high fever followed by a rash appearing with fever subsidence (34, 39). Thus, the frequency of HHV-6B seropositive individuals reaches almost 100% in early childhood (7, 10), but late primary infection may provoke a mononucleosislike disease similar to human cytomegalovirus (HCMV) mononucleosis. In addition, HHV-6B is thought to be involved in a number of other conditions, including encephalitis and neoplasia (2, 3).HHV-6B displays primarily tropism for T cells and monocytes (22,24). The virus enters the cells in part via binding to the ubiquitously expressed glycoprotein CD46 (33), but additional cellular proteins involved in virus entry still remain to be discovered (6, 33). As with other herpesviruses, HHV-6 establishes latency following the primary infection. In addition to its tropism for mononuclear cells, HHV-6 is highly neurotropic and may also reside in astrocytes within the central nervous system (8, 15), and it remains to be determined which cells harbor the majority of the latent virus (21, 23). Of clinical significance, HHV-6 is an important pathogen when reactivated in immunocompromised hosts, such as patients with AIDS, leukemia, lymphoma, and solid organ or bone marrow transplants (3).The entire genomes of HHV-6B strains Z29 and HST have...