Inherited long QT syndrome (LQTS) is caused by mutations in six genes including SCN5A, encoding the α-subunit of the human cardiac voltage-dependent sodium channel hNa_v1.5. In LQT3, various mutations in SCN5A were identified, which produce a gain of channel function. The aim of this study was to screen SCN5A for mutations in a family with the LQT3 phenotype and to analyze the consequences of the mutation on the channel function. By polymerase chain reaction-denaturating high performance liquid chromatography-sequencing, we identified a novel deletion in SCN5A, delQKP 1507–1509, in the DIII–DIV linker of the sodium channel. The hNa_v1.5/delQKP1507–1509, hNa_v1.5/delQ1507 and hNa_v1.5/Q1507A mutants were constructed in vitro, mutant channels were expressed in the tsA201 human cell line and studied using the whole-cell configuration of the patch clamp technique. A persistent inward sodium current of 1–1.5% of maximum currents measured at −30 mV in all mutant sodium channels was recorded, which was nearly completely blocked by the sodium-channel blockers tetrodotoxin and lidocaine. The deletion mutants resulted in a significant shift of steady-state activation to more depolarized voltages. The delQ1507 showed a small shift of steady-state inactivation towards more negative potentials, whereas no significant shifts were observed in both steady-state activation and inactivation in Q1507A compared to the wild-type Na_v1.5 sodium channels. The novel SCN5A mutation, delQKP, induces a residual current as previously shown for other SCN5A mutations causing LQTS. DelQKP shares the deletion of Q1507 with the formerly known delKPQ 1505–1507. Our data suggest that Q1507 plays an important role in fast sodium channel inactivation.