Deep sequencing was used to bring high resolution to the human cytomegalovirus (HCMV) transcriptome at the stage when infectious virion production is under way, and major findings were confirmed by extensive experimentation using conventional techniques. The majority (65.1%) of polyadenylated viral RNA transcription is committed to producing four noncoding transcripts (RNA2.7, RNA1.2, RNA4.9, and RNA5.0) that do not substantially overlap designated protein-coding regions. Additional noncoding RNAs that are transcribed antisense to protein-coding regions map throughout the genome and account for 8.7% of transcription from these regions. RNA splicing is more common than recognized previously, which was evidenced by the identification of 229 potential donor and 132 acceptor sites, and it affects 58 proteincoding genes. The great majority (94) of 96 splice junctions most abundantly represented in the deep-sequencing data was confirmed by RT-PCR or RACE or supported by involvement in alternative splicing. Alternative splicing is frequent and particularly evident in four genes (RL8A, UL74A, UL124, and UL150A) that are transcribed by splicing from any one of many upstream exons. The analysis also resulted in the annotation of four previously unrecognized protein-coding regions (RL8A, RL9A, UL150A, and US33A), and expression of the UL150A protein was shown in the context of HCMV infection. The overall conclusion, that HCMV transcription is complex and multifaceted, has implications for the potential sophistication of virus functionality during infection. The study also illustrates the key contribution that deep sequencing can make to the genomics of nuclear DNA viruses.T he genetic repertoire of human cytomegalovirus (HCMV; species Human herpesvirus 5) is incompletely understood. Most bioinformatic investigations have focused on identifying open reading frames (ORFs) that are conserved in other organisms or that exhibit pattern-based similarities (e.g., in nucleotide or codon bias) to recognized protein-coding regions (CRs) (1). Our current map of the wild-type HCMV genome, based on strain Merlin, contains 166 protein-coding genes (2-5). It is entirely possible that additional, small protein-coding genes will be found. Candidates involve ORFs that overlap recognized CRs and for which there is some evidence for expression (6), ORFs highlighted in pattern-based bioinformatic (7) or proteomic analyses (8), and ORFs whose expression is presently unsuspected.Recognition of many protein-coding genes has been supplemented by information on protein expression and function. However, HCMV also specifies polyadenylated (polyA) transcripts that, because they lack sizeable, conserved ORFs, appear unlikely to function via translation. One class consists of noncoding, nonoverlapping transcripts (NNTs) that do not substantially overlap the designated CRs of other genes. These include an abundant 2.7-kb RNA (β2.7 or RNA2.7) (9), a 1.1-kb spliced RNA and associated 5-kb stable intron (RNA5.0) (10, 11), and a 1.2-kb RNA (RNA1.2) (12). RNA...