Protein structure information from X-ray crystallography and bioinformatics has given biochemists a potentially potent anticancer target: specific segments of DNA's double helix. But, as a group of biochemists at a fall conference sponsored by the National Foundation for Cancer Research agreed, knowing the structure of proteins and protein complexes that bind to DNA segments and turn genes off and on may be only half the battle. Proteins or large synthetic molecules should, in theory, be able to repair defective genes whose malfunctions give rise to tumors. But large molecules degrade in the body. And delivery could prove difficult.