Several companies and medical laboratories are developing rapid genetic tests to determine who is likely to respond poorly, or not at all, to standard doses of common medications. These tests represent the beginning of a fundamental change in the way medicine is practiced, by tailoring the choice of drug and the amount prescribed to the genetics of the patient.

The full impact may not be felt for five or 10 years. But already, Roche, the worldwide leader in marketing diagnostic tests, is betting $70 million on new gene chips from Affymetrix in Santa Clara. The first Roche AmpliChip will check patients for common variations in two genes that play crucial roles in the breakdown of almost half of the prescription drugs now on the market.

The two companies hope that by combining forces they can become leaders in the emerging field of “pharmacogenetics” — the science of predicting drug reactions by measuring individual genetic differences.

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