Exhibit 99.1 Design Therapeutics Announces Plans to Initiate Patient Dosing of DT-818 in Myotonic Dystrophy Type-1 (DM1) in the First Half of 2026 and Reports Third Quarter 2025 Financial Results Obtained ex-US Regulatory Clearance for DT-818, a Potentially Best-in-Disease Treatment for Myotonic Dystrophy Type-1 (DM1) Trials of DT-216P2 in Friedreich Ataxia (FA) and DT-168 in Fuchs Endothelial Corneal Dystrophy (FECD) Ongoing Cash and Securities of $206.0 Million as of Third Quarter 2025 Support Continued P ...