these sister s has have denoite their brains to the study of alzheimer's, a disease common in aging and especially those over 65. have any didn't know how the sisters would react to it, but when they heard of the explanationk and how it would help people, and help other people, the sisters were all for it. >> and my spirit has are rejoiced in my savior. >> i remember one sister saying, of course, i will do this. what good is it going to do me when my brain is six feet under. >> reporter: alzheimer's is the most common cause of dementia, and degenerative neur lological disease. the figure of people diagnosed with the disease is expected to double. of the drug treatments in development, none of them offer a u cure. but what also are remains unclear is why we get it. >> it was designed as an autopsy study to follow a age cohort of sister, and so a group of s sisters all born around the same time. the goal of the study was to collect the brains at a autopsy and to try to correlate day-to-day functioning during life with the type and level of burden of lesions on the brain after death. these