CHICAGO: Drugmakers including Bristol-Myers Squibb Co and Merck & Co are testing which patients will most benefit from new cancer treatments based on a protein found in their tumors, but that guide, known as a biomarker, may be too unreliable, researchers and health experts said. Bristol's Opdivo and Merck's Keytruda are both therapies designed to block a protein known as Programmed Death receptor that tumors use to evade the body's natural defenses.