Here, we argue that the loss of neurons and proteins are the most robust features of neurodegeneration. For instance, compared to healthy controls, the levels of alpha‐synuclein (α‐syn) in cerebrospinal fluid (CSF) are low in Parkinson's disease (PD) [12, 13], and the levels of 42‐amino acid amyloid‐beta (Aβ42) are low in AD [14]. This evidence concerns the gene SNCA and Parkinson disease.