Given the demonstration here of activated STAT3 in renal macrophages of ADPKD cystic kidneys (Fig. 6; Fig. S4), we think it likely that the unidentified phospho-STAT3-positive interstitial cells in the cystic kidneys noted in all those previous studies were macrophages. The gene discussed is STAT3; the disease is autosomal dominant polycystic kidney disease.