We propose to develop methods for preparing libraries of cDNA fragments derived from sequences adjacent to the cap site of mRNAs. These methods will be applied to generate 5' end cDNA libraries from a number of human sources. Methods will be developed to select directly from these libraries those cDNAs encoded by large DNA fragments such as YACs without the need to purify the YAC DNA from the host yeast sequences. These methods would allow selection of cDNAs from arrays of YACs. In addition, the usefulness of the 5' end libraries for obtaining promoter sequences and preparing full-length cDNAs will be evaluated. These libraries will provide an improved approach for detecting expressed sequences and positional cloning of disease genes.