Question: Have scientists identified the gene(s) that cause Alzheimer's disease?
Answer: Over the past two decades we have identified four different genes that influence risk for Alzheimer's disease. Three of them are involved with the early-onset familial form, which strikes under 60 years old and clusters within families. Those contain mutations that are very robust, that guarantee the disease when inherited, but they're very rare.
Most cases of Alzheimer's are late-onset, striking after 60 years old, and there we know about one confirmed risk factor called the ApoE gene, which just increases risk but doesn't guarantee disease.
Meanwhile, efforts are ongoing in labs around the world to identify the remaining Alzheimer's disease genes.