A perfect prescription would fix what ails you, and leave the rest of you well enough alone. That's the ideal. The reality: Side effects bedevil almost all available drugs, and keep many others from ever reaching the market. A new approach for monitoring drugs' consequences in yeast cells could help sort the silver bullets from bombs more efficiently. Acacia Biosciences-a startup in Richmond, Calif.-is systematically "knocking out," or disabling, each yeast gene and studying what effect the loss of that specific gene has on the cell. The readout serves as a "molecular fingerprint of what the perfect drug would do," says Acacia CEO Bruce Cohen, since such a therapeutic agent would block the function of one-and only one-gene.

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