The drug so far has only helped patients whose tumors have the BRAF mutation.

FORBES: American Society of Clinical Oncology

Prof Richard Marais, from the Paterson Institute for Cancer Research in Manchester, was involved in the discovery of the BRAF fault.

BBC: 'Drug holidays' beat cancer drug resistance in mice

In 2002 gene researchers in the U.K. discovered that two-thirds of melanomas have a mutation in a growth-promoting gene inside skin cells called BRAF.

FORBES: Targeting Melanoma

The Roche braf drug kicks in quickly, which can help symptoms, while the Bristol-Myers drug takes months to act, only occasionally produces big tumor shinkage.

FORBES: The Cancer Trial Bristol and Roche Must Do Now

The mutation causes the BRAF protein to become stuck in the "on" position, so it constantly sends a signal to the nucleus that it is time to proliferate.

FORBES: Targeting Melanoma

Roche's drug, called PLX4032, targets a mutation in a gene called BRAF that appears to be an important growth driver in 60% of melanoma cases and is also found less frequently in many other cancers.

FORBES: Magazine Article

Vemurafenib targets a mutated protein called BRAF.

BBC: 'Drug holidays' beat cancer drug resistance in mice

Meanwhile, PLX4032 from Roche and Plexxikon works via a different mechanism and shrinks tumors in a mind-boggling 80% of melanoma patients whose tumors have a mutation called in a gene called BRAF, These results were published this week in the New England Journal of Medicine.

FORBES: The Cancer Trial Bristol and Roche Must Do Now