To explore the effect of the mutated PAS on DUX4 steady-state transcript levels, we carried out base editing in FSHD patient-derived immortalized myoblasts, since HAP1 cells do not express DUX4. We used three different FSHD myogenic cell lines with different genetic characteristics, D4Z4 methylation status and DUX4 expression levels (Figures S2A and S2B). Here, DUX4 is linked to Facioscapulohumeral dystrophy.