Alternative splicing of pre-mRNAs is an important mechanism for the generation of vertebrate protein diversity. Unfortunately, the contribution of alternative splicing to protein diversity is currently not well understood because many full-length mRNA sequences have yet to be identified. In this report, we describe the use of RT-PCR to identify and clone 279 novel alternatively spliced mRNAs from 114 well-known drug target genes. Our findings demonstrate the existence of many novel alternatively spliced mRNA transcripts and suggest that many more genes undergo functionally significant alternative splicing than previously thought.