Twin 4-year-old girls Addison and Cassidy Hempel are similar to their
contemporaries in many ways: They love nursery rhymes; "Humpty Dumpty"
is the family favorite; they go to school and play house.
Meet the parents on a quest for a cure.

But the identical siblings differ from other children in one major
way. The girls' world of childhood fantasy is haunted by a
heartbreaking reality that they are fighting a disease that seeks to
steal their memories.

Like a pediatric version of Alzheimer's disease, Niemann-Pick Disease
Type C (NPC)...........read more here

http://www.abcnews.go.com/GMA/OnCall/story?id=4760023&page=1