Therefore, the presence of STAT3 mutations would contribute to a more robust demonstration of clonality and diagnosis of T/NK-LGLL, particularly in cases where oligoclonal LGL expansions are observed on phenotypic grounds, and in CLPD-NK patients in whom there are no alternative techniques for confirmation of NK-cell clonality, as shown here. This evidence concerns the gene STAT3 and T-cell large granular lymphocyte leukemia.