STAT3 and pulmonary arterial hypertension: During receptor-mediated endocytosis, Tyr705-phospshorylated STAT3 colocalizes with endocytic vesicles in transit from the cell membrane to the perinuclear region.46–48 In pulmonary arterial hypertension, this centripetal trafficking of STAT3 is disturbed in pulmonary arterial cells, where STAT3 and P-Tyr705-STAT3 are sequestered inside the lysosomes.49 In our experiments employing untreated cancer cells, STAT3 did not colocalize with early endosomes, and the majority of lysosomal STAT3 was not phosphorylated at Tyr705 residue.