When molecular biologists began analyzing the complete sequence of the human genome in mid-2001, one surprising observation was that humans have relatively few genes. We may have as few as 30,000 genes, only about two times as many as the much simpler fruit fly, Drosophila melanogaster. How can the much greater size and complexity of humans be encoded in only twice the number of genes required by a fly? The answer to this paradox is not fully understood, but it appears that humans and other mammals may be more adept than other organisms at encoding many different proteins from each gene. One way they do this is through alternative splicing, the processing of a single RNA transcript to generate more than one type of protein.

Introns are removed by the spliceosomes.

In most eukaryotic genes, the protein-coding sequences, termed exons, are interrupted by stretches of sequence, termed...