GFAP and Parkinson disease: Certainly, an additional noteworthy aspect to be highlighted in this investigation is that through subgroup analysis considering age, gender, clinical cognitive diagnosis, APOE ε4 allele, and Aβ status, respectively, we discovered that the predictive value of GFAP baseline level on cognitive decline in PD was significant but there were subtle differences, which were manifested in different cognitive domains as well as cognitive severity.