(Reissue PAR-21-056) The purpose of this Program Announcement (PAR) is to enable analytical validation of strong candidate biomarkers for neurological diseases and conditions. Specifically, the goal of this PAR is to enable the rigorous validation of analytical methods for biomarker measurements, including evaluation of the detection method, its performance characteristics, and the optimal conditions that will generate reproducibility and accuracy consistent with FDA guidelines. This PAR assumes that 1) a candidate biomarker has already been identified, 2) detection method technology has already been developed, and 3) the research and/or clinical need and potential context of use has been identified.