Five years ago, little was known about mechanisms of apoptotic executi
on. Now, one class of cell-death gene, the cysteine aspartases (caspas
es) has come under intensive study. This review discusses the two clas
ses of caspases, the reasons why humans may have so many caspase genes
, the growing list of caspase substrates, and viral and pharmacologica
l caspase inhibitors.