Representative histological features of pediatric cases with (A) ST, (B) ET, and (C) pre-PMF. (A) Normocellular BM with scattered nondescript MK. (B) Loose clusters of MKs with hypersegmented (staghorn-like) nuclei. (C) Tight clusters of atypical MKs with bulbous (cloud-like) nuclei. Hematoxylin and eosin stain, original magnification ×10 and ×20; Leica DM4000 B optic microscope, DFC420 camera and acquisition software (Leica Microsystems, Milan, Italy).