Long-QT syndrome (LQTS) is a genetic disorder characterized by prolonged QT intervals on ECG and symptoms such as syncope, seizures, and sudden death. Five genes—KVLQT1, HERG, SCN5A, KCNE1, and KCNE2—are associated with LQTS. Mutations in KVLQT1 and KCNE1 also cause Jervell and Lange-Nielsen syndrome, which is linked to deafness. This study analyzed 262 unrelated individuals with LQTS for mutations in these five genes, identifying 134 mutations, 80 of which were novel, bringing the total to 177 mutations (68% of individuals). KVLQT1 (42%) and HERG (45%) accounted for 87% of mutations, while SCN5A (8%), KCNE1 (3%), and KCNE2 (2%) accounted for the remaining 13%. Most mutations were missense (72%), followed by frameshift (10%), in-frame deletions, and nonsense/splice-site mutations (5-7% each). Mutations were predominantly in intracellular (52%) and transmembrane (30%) domains. Most mutations were found in single families or individuals. The study highlights the importance of genetic screening for LQTS diagnosis and treatment. Mutations in KVLQT1, HERG, KCNE1, and KCNE2 may benefit from potassium therapy, while SCN5A mutations may benefit from sodium channel blockers. The study also contributes to understanding ion channel function and disease mechanisms. This catalog of mutations aids in genotype-phenotype analysis and genetic screening for arrhythmia susceptibility.Long-QT syndrome (LQTS) is a genetic disorder characterized by prolonged QT intervals on ECG and symptoms such as syncope, seizures, and sudden death. Five genes—KVLQT1, HERG, SCN5A, KCNE1, and KCNE2—are associated with LQTS. Mutations in KVLQT1 and KCNE1 also cause Jervell and Lange-Nielsen syndrome, which is linked to deafness. This study analyzed 262 unrelated individuals with LQTS for mutations in these five genes, identifying 134 mutations, 80 of which were novel, bringing the total to 177 mutations (68% of individuals). KVLQT1 (42%) and HERG (45%) accounted for 87% of mutations, while SCN5A (8%), KCNE1 (3%), and KCNE2 (2%) accounted for the remaining 13%. Most mutations were missense (72%), followed by frameshift (10%), in-frame deletions, and nonsense/splice-site mutations (5-7% each). Mutations were predominantly in intracellular (52%) and transmembrane (30%) domains. Most mutations were found in single families or individuals. The study highlights the importance of genetic screening for LQTS diagnosis and treatment. Mutations in KVLQT1, HERG, KCNE1, and KCNE2 may benefit from potassium therapy, while SCN5A mutations may benefit from sodium channel blockers. The study also contributes to understanding ion channel function and disease mechanisms. This catalog of mutations aids in genotype-phenotype analysis and genetic screening for arrhythmia susceptibility.