How many human genes are expressed ubiquitously, in all human tissues, and how many are expressed in tissue-specific patterns? To answer these fundamental questions in molecular biology, we have analysed 3.5-million transcripts from 19 normal and diseased tissue types. We found that as many as 43,500 genes can be expressed in a single cell type. Only a small fraction of transcripts were exclusively expressed in any individual tissue, whereas nearly 1,000 genes were expressed in all cell types examined. We found 40 genes to be expressed at elevated levels in all cancer tissues but not in normal cells. Serial analysis of gene expression (SAGE) studies1, 2 of 84 libraries derived from 19 different sources identified 134,135 transcripts from approximately 84,000 different genes (Table 1; data and analysis available at http://genetics. nature. com/supplementary_info/). Expression levels for these genes ranged from 0.3 to 9,417 transcript copies per cell. The transcript tags matched approximately 4,300 known genes and 41,000 genes with unknown functions, whereas the remaining transcript tags (46%) had no matches to existing databases (Table 2, see http://genetics. nature. com/supplementary_info/). The subset of expression data from colorectal cancer cell lines provided the first relatively complete analysis of the transcripts expressed in a single mammalian cell type. We analysed 643,283 transcripts from colorectal cancer cell lines. As human cells contain approximately 300,000 mRNA molecules, this number was sufficient to provide approximately twofold coverage of the transcriptome, revealing over 83% of transcripts expected to be pre-