CLU and Parkinson disease: Two-dimensional liquid chromatography–tandem mass spectrometry (2D-LC-MS/MS) coupled with isobaric tags for relative and absolute quantification (iTRAQ) labeling was used by Zhang et al. [35] to demonstrate higher clusterin concentration in serum of PD patients as compared to controls, with more pronounced elevation in stages I–II (by 2.8-fold) than III–IV (by 1.4-fold) according to Hoehn and Yahr classification.