Using a new mass spectrometry-based technique (MRM) for the quantitation of peptides and proteins in the CSF of our ALS patients, we were able to identify other proteins like cathepsin D (CTSD), thrombin (F2), plasminogen (PLG) and secretogranin III (SCG3) as key players in the biomarker network (Figure 4). Here, CTSD is linked to amyotrophic lateral sclerosis.