HPRT1 and amyotrophic lateral sclerosis: Figure 1D shows that the HPRT mutation rate in iPSCs containing TDP-43(M337V) or TDP-43(Q343R) (hereafter called mutant TDP-43 ALS iPSCs) was substantially higher than that observed in control iPSCs, indicating increased genomic instability in ALS patient cells harboring TDP-43 mutations.