Membrane proteins have proved very difficult to crystallize, with only a single example of a G protein-coupled receptor structure in the public domain. Indeed, getting crystals is the first and most difficult step on the road to obtaining a protein's structure. Thus, the belief that 3-Dimensional Pharmaceuticals Inc. had produced the crystals of a second GPCR promised significant improvement in the ability of companies to do structure-based drug design on a key family of proteins. But earlier this month DDDP announced that it did not have the crystals, after all.

GPCRs make up the largest family of validated drug targets - about 60 percent of approved drugs act at GPCRs. GPCRs are such prominent drug targets because they are responsible for most cellular signaling and they make up about 3 percent of the human