Molecular biologists are increasingly faced with the problem of assigning a function to genes that have been cloned. A new approach to this problem involves the manipulation of the cloned gene to create what are known as 'dominant negative' mutations. These encode mutant polypeptides that when overexpressed disrupt the activity of the wild-type gene. There are many precedents for this kind of behaviour in the literature--some oncogenes might be examples of naturally occurring dominant negative mutations.