Updated publication reference for PubMed record(s): 27757297. STAT1 is an important regulator of NK cell maturation and cytotoxicity. Although the consequences of Stat1-deficiency have been described in detail the underlying molecular functions of STAT1 in NK cells are only partially understood. Here we describe a novel non-canonical role of STAT1 that was unmasked in NK cells expressing Stat1-Y701F. This mutation prevents JAK-dependent phosphorylation, subsequent nuclear translocation and cytokine-induced transcriptional activity. As expected Stat1-Y701F mice displayed impaired NK cell maturation comparable to Stat1-/- animals. In contrast Stat1-Y701F NK cells exerted a significantly enhanced cytotoxicity in vitro and in vivo suggesting a so-far unknown cytoplasmic function. Using immunofluorescence technology we uncovered the recruitment of STAT1 to the immunological synapse during NK cell killing. A Stat1ind mouse expressing FLAG-tagged STAT1α was used to study the STAT1α interactome in NK cells. Mass spectrometry revealed that STAT1 directly binds proteins involved in cell junction formation and proteins associated to membrane or membrane-bound vesicles. We propose a novel function for STAT1 in the immunological synapse of NK cells regulating tumor surveillance and cytotoxicity.