cDNA analyses in the human genome project.

The ultimate goal of the human genome project is to decode all the genetic information carried in the genome. Towards this goal, the physical structure of the genome, as well as the functional aspects of the genome, must be understood. We initiated a cDNA project to collect the 'expression profiles' of all human genes, a database with which to describe which genes are expressed, and to what extent, in any given human cell at a particular time. Single-cycle sequencing of randomly selected members from a 3'-directed cDNA library is most appropriate for this purpose: the sequence data serve as a 'gene signature' to identify the expressing gene, and the frequency of appearance of the gene signature reflects the activity of the gene. The compiled data, which usually cover some 1000 sequencing results per sample, are referred to as an 'expression profile.' We applied this analysis to HepG2 (a cell line derived from a hepatocellular carcinoma), liver cells and lung cells. The expression profiles shed some light upon the unique features of gene expression in the cell or tissue tested. A comparison of the expression profiles among different cells has allowed active genes to be classified as housekeepers or those with cell-specific functions. A significant fraction of the abundantly expressed genes include those that are unique to the cell. In addition, the resulting collection of thousands of gene signatures is a useful source of probes for mapping and for isolating full-size cDNAs.