Alternative splicing is the major source of proteome diversity in humans and thus is highly relevant to disease and therapy. For example, recent work suggests that the long-sought-after target of the analgesic acetaminophen is a neural-specific, alternatively spliced isoform of cyclooxygenase 1 (COX-1). Several important diseases, such as cystic fibrosis, have been linked with mutations or variations in either cis-acting elements or trans-acting factors that lead to aberrant splicing and abnormal protein production. Correction of erroneous splicing is thus an important goal of molecular therapies. Recent experiments have used modified oligonucleotides to inhibit cryptic exons or to activate exons weakened by mutations, suggesting that these reagents could eventually lead to effective therapies.