The antibodies bind to the CTLA-4, found on a cell's surface, and shut off the brake.

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In the mid-1990s he theorized that CTLA-4 might prevent the immune system from mounting an effective response against tumors.

FORBES: Targeting Melanoma

But Allison showed in 1996 that he could shrink tumors in mice by injecting them with antibodies to CTLA-4.

FORBES: Targeting Melanoma

UCLA's Ribas calls the response rates "very low" and cautions that the anti-CTLA-4 drugs are just a first step.

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To improve this situation, researchers are combining new antimelanoma vaccines with anti-CTLA-4 drugs.

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Bristol's drug Yervoy inhibits the function of a molecule called CTLA-4.

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Both Pfizer and Medarex, a biotech firm in Princeton, New Jersey, subsequently produced human antibodies to CTLA-4 and began testing them in patients a few years later.

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Pfizer's drug tremelimumab and Bristol-Myers Squibb's ipilimumab are antibodies to a protein called CTLA-4 (cytotoxic T-lymphocyte antigen-4) that acts as an emergency brake to prevent killer T cells from attacking healthy tissue.

FORBES: Targeting Melanoma

Both Pfizer and Medarex (nasdaq: MEDX - news - people ), a biotech firm in Princeton, New Jersey, subsequently produced human antibodies to CTLA-4 and began testing them in patients a few years later.

FORBES: Targeting Melanoma