A survey method of gene expression analysis
A cDNA library (or libraries) for the tissue and state of interest is located. We make the assumption that the abundance of a cDNA in that library is a good reflection of its abundance in the tissue and state it was harvested from. A large number of clones are selected at random from the library, sequenced, and the gene/s contained within each clone identified by database searches (GenBank etc.).
We can apply statistical tests to determine if our sample is large enough to be statistically robust ie. we have a good estimate of relative mRNA expression levels. To increase our resolution and determine the relative levels of rarer mRNA transcripts we simply take more samples...though the sequencing effort involved becomes prohibitive (but see SAGE and MPSS).
The random sequencing of fragments of cDNA direct from tissues (rather than sampling from a library) is a popular new paradigm. Database searches involving these expressed sequence tags (ESTs) can also be indicative of relative gene expression levels.