U2OS cells transfected with Flag-FLT3-ITD, Flag-FLT3, p27 or p27-Y88F were analyzed as described.13 (link) Briefly, cells were fixed with 4% para-formaldehyde, permeabilized with 0.025% saponine, and blocked with 0.5% gelatine. A Leica DMi8 inverted widefield microscope was used for imaging.