are related genes implicated in early-onset Alzheimer's. Presenilin 1 (PS1) is found on chromosome 14, and PS2 on chromosome 1. Presenilins are involved in the production of amyloid peptides, but exactly how mutations in the genes cause Alzheimer's is not yet understood. Mouse studies have found that deletion of these genes causes memory loss and gradual death of nerve cells in the mouse brain, demonstrating that the protein products of these genes are essential for normal learning, memory and nerve cell survival.