Frequency and type of JAK2 V617F mutation within diagnostic categories. (G/G: wild type; G/T or T/T: heterozygous or homozygous for mutation) (left panel). Within WHO MDS/MPD-U, 6 of 9 cases that were positive for JAK2 V617F mutation occurred in RARS-T. When separated from MDS/MPD-U, 67% of RARS-T cases showed a JAK2 V617F mutation, but only 12% of the remaining MDS-MPD-U were positive (right panel).