Immunohistochemical detection of p-STAT. Positive staining is present in nuclei of megakaryocytes and erythroid precursors (A) in a patient (Table 3, Patient no. 7) with RARS-T heterozygous for JAK2 V617F mutation (G/T). Negative antibody control (B). Positive staining is present in nuclei of erythroid precursors but absent in megakaryocytes (C) in a patient (Table 3, Patient no. 1) with RARS-T who lacks JAK2 V617F mutation. Negative antibody control (D). Original magnification, 100×.