GRN and frontotemporal dementia: For example, reduced serum PGRN do not seem to correlate tightly with reduced PGRN levels in the CSF of both Pgrn ± and Pgrn−/− FTD patients, suggesting either differences in peripheral PGRN regulation or potential sources of measurement error in PGRN quantification between these sample types (Wilke et al., 2016).