1 Department of Infection, Division of Infection and Immunity, Royal Free and University College Medical School of UCL, London, UK 2 Virus Genomics Team, Wellcome Trust Sanger Institute, Cambridge, UK Correspondence Duncan A. Clark duncan.clark{at}bartsandthelondon.nhs.uk The lytic gene expression of several members of the human herpesvirus family has been profiled by using gene-expression microarrays; however, the lytic cascade of roseoloviruses has not been studied in similar depth. Based on the complete DNA genome sequences of human herpesvirus 6 variant A (HHV-6A) and variant B (HHV-6B), we constructed a cDNA microarray containing DNA probes to their predicted open reading frames, plus 914 human genes. Gene-expression profiling of HHV-6B strain Z29 in SupT1 cells over a 60 h time-course post-infection, together with kinetic classification of the HHV-6B genes in the presence of either cycloheximide or phosphonoacetic acid, allowed the placement of HHV-6B genes into defined kinetic classes. Eighty-nine HHV-6B genes were divided into four different expression kinetic classes: eight immediate-early, 44 early, 33 late and four biphasic. Clustering of genes with similar expression profiles implied a shared function, thus revealing possible roles of previously uncharacterized HHV-6B genes. The ArrayExpress accession numbers for the microarray data generated in this paper are E-MEXP-2129 and E-MEXP-2130. Supplementary methods, figures, tables and references are available with the online version of this paper.