Additional Study Information: The purpose of this Phase I study is to see whether a single dose of an experimental drug (meaning a drugthat is not approved by the FDA) called C2N-8E12 given into your vein (called intravenous or IV) is safeand well-tolerated in subjects with Progressive Supranuclear Palsy (PSP). C2N-8E12 is an immunotherapy (antibody) drug designed to bind to and to remove tau protein. The tau protein is believed to build up to abnormally high levels in the brains of individuals with PSP. This abnormal accumulation is associated with clinical progression of the disease. In addition, this study will also test the effects of C2N-8E12 on cerebrospinal fluid (CSF) and certain blood proteins (including tau) in subjects with PSP. This study uses a placebo that looks like the experimental drug but does not have any active drug in it. This means that 1 out of every 4 subjects (or 25%) who take part in this study will not receive active treatment from their participation.