PSP is a brain disease in the category of “neurodegenerative” diseases along with Alzheimer’s, Parkinson’s, and ALS, also known as Lou Gehrig’s disease. In PSP, cells in some areas of the brain accumulate clumps of a protein called tau and gradually die off. Those cells control walking, balance, mobility, vision, speech, swallowing, sleep, and behavior.

Each year an average of 1.1 people per 100,000 In the U.S. are newly diagnosed with PSP; five or six people per 100,000 are living with the disease. These figures are nearly identical wherever they have been carefully measured, which is in only three countries—the U.K., the U.S., and Japan.

PSP’s average age of onset is 63, but rare cases have started in the early 40s. It is slightly more common in men than women, but PSP has no known geographical, occupational, or racial preference. PSP is often misdiagnosed as Parkinson’s disease because of the general slowing of movement.