Thanks to innovative chemical treatments, the ball serves as a cage that hides the enzyme from the immune system until it gets to its target tumor, where an anti-cancer drug is released.

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Researchers are trying, and company-funded clinical trials of targeted drugs have at least looked at whether the target is expressed (turned on) in tumor tissues.

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Every tumor has five to ten flawed genes driving its growth, and researchers may have to target several simultaneously with cocktails of drugs, the approach that worked against AIDS. But finding such cocktails will be slow and difficult because regulators frown on testing two experimental drugs together, and corporate rivals rarely share drugs.

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Recent examples: molecules to target drug-resistant bacteria and a kinase inhibitor to zap the enzyme responsible for tumor growth.

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Instead of aiming at broad tumor categories, such as all patients with lung cancer or colon cancer, the new medicines target specific subsets of patients whose tumors have particular gene mutations.

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