TK1 and frontotemporal dementia: The relationship between TK1 knock-out and cytotoxicity of FTD was described above (Fig. 1); however, it was unknown whether FTD cytotoxicity correlates with the expression level of TK1. To address this question, we first examined FTD cytotoxicity in TK1-overexpressing RKO cells, in which basal expression of TK1 was low (Fig. S1A).