To confirm the presence of ATM gene mutations, we assessed ATM protein expression by western blot (Figures 1D, S1A, and S1B). A band corresponding to full-length ATM protein was clearly visible in the CAR3 and Q1SA iPSC lines, as expected (Figure S1A). Q1SA encodes a missense mutation (c.7181 C > T; encoding S2394L, Figure 1A) that is predicted to affect ATM activity but not translation of full-length protein. However, independent sublines prepared from subject JHU_Q3 expressed unexpected variation in ATM expression (Figures 1D and S1B). Some sublines had no ATM protein, as expected (sublines Q3SA, Q3SE, and Q3SI), but others expressed full-length protein (Q3SC and Q3SG). Furthermore, different passage numbers from the same subline exhibited varying quantities of ATM protein, peaking at P7 and then diminishing but not disappearing in later passages (Q3SC; Figures 1D and S1B). We selected two sublines, Q3SA and Q3SC, as contrasting examples without or with ATM expression, respectively. ATM activity in sample iPSC lines was confirmed by the X-irradiation (XR)-induced phosphorylation of CHK2 as detected by western blots (Figure 1E). The pCHK2 band was enhanced
