Abstract: The present invention provides novel methods for identifying gene expression patterns in mRNA populations. The methods are useful for determining differential gene expression among various cells or tissues, including cells or tissues of a target organism. The invention also provides methods of determining the frequency of gene expression in mRNA populations, thus providing a method of comparing gene expression frequency among various cells or tissues. The present invention also provides methods for isolating genes corresponding to tag sequences identified according to the methods of the present invention. Furthermore, sequences that are identified according to the present invention may be used to diagnose the presence of disease.