June Ward (right) — pictured with her sisters Susie Gilliam (center) and Karen Douthitt (left) — carries a rare gene mutation that virtually guarantees she will get Alzheimer's in the next few years. She is part of a network of more than 200 families with these kinds of gene mutations who have volunteered as research subjects over the past two decades. Juan Diego Reyes for NPR hide caption

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Juan Diego Reyes for NPR

Some of the most important studies of potential treatments for Alzheimer's disease rely on a group of participants who know they may never fully reap the benefits.

"It's not for us," says June Ward, 64, who carries a rare gene mutation that virtually guarantees she will get Alzheimer's in the next few years.…