Immunohistochemical staining of phosphorylated STAT3 and PD-L1 in NKTL tumor samples with STAT3 mutations. (A-F) Formalin-fixed, paraffin-embedded sections from tumor biopsy samples of patients with NKTL with novel (p.E616G, p.E616K, p.E696K) and hotspot (p.S614R, p. G618R, p.Y640F) STAT3 mutations that were stained with antibodies against pSTAT3 (left) and membranous PD-L1 (right). Immunohistochemistry shows constitutive phosphorylation and nuclear localization of STAT3 (Y705) associated with high PD-L1 expression in the STAT3 mutant cases (brown staining). Original magnification, ×400.