Figure 4A shows the absolute frequency of the top 25 most used keywords and their relative frequencies over the periods. The frequency of occurrence of most research topics, such as neurodegeneration, frontotemporal dementia, and Alzheimer’s disease, had increased. There had been explosive growth in the use of some keywords since their debuts, such as TDP-43, FUS, and C9orf72. The frequency of use of other keywords, such as neuroprotection, apoptosis, and excitotoxicity, decreased. Figure 4B shows the proportion dynamics for each keyword. Here, C9orf72 is linked to frontotemporal dementia.