In the article, “Tau forms in CSF as a reliable biomarker for progressive supranuclear palsy,” Dr. Borroni and her coauthors ( Neurology ®2008;71:1796–1803) looked at specific medical tests that could help physicians accurately diagnose this illness. Progressive supranuclear palsy (PSP) is rare and has similarities to several other progressive neurologic disorders. Because of this, they looked at the results of these tests not only in people with PSP, but also in those who had the “similar” illnesses. In addition, the authors compared the results of the tests to test results of people who did not have progressive neurologic disease. Dr. Borroni and coauthors carefully studied the tests results for 166 people. All had undergone a detailed history, physical examination, and neurologic examination. All were monitored for more than 2 years. The reason for this long period of monitoring is that the illnesses, especially when they begin, can look like something else. As the illness progresses and other symptoms develop, the diagnosis becomes more clear. By following this group for more than 2 years, Dr. Borroni minimized any errors in diagnosis. Of the 166 people, 21 were diagnosed with PSP. One hundred twenty-five people had progressive neurologic conditions such as Parkinson disease and Alzheimer disease. There were also a few other illnesses. Twenty-seven patients acted as the “controls.” They were “normal.” This last group needs more explaining. The “controls” were a group of people who had an MRI and lumbar puncture for other reasons. For instance, they may have had a headache, and in the process of evaluating the headache, these tests were needed. Although not entirely “normal,” as this group had experienced a neurologic symptom (for example, headache), they did not have a progressive neurologic illness. A subset of patients had a specialized MRI of the brain and brainstem. Dr. Borroni …