GSTM1 and Splenomegaly: We then investigated whether the homozygosity for the SNP could impact the disease outcome, assessing whether PMF harboring the G/G genotype have a higher chance of incurring in: (i) severe anemia (Hb < 100 g/L), massive splenomegaly (≥10 cm below costal margin) or leukocytosis (WBC ≥ 12 × 109/L); (ii) blast transformation (defined as a proportion of >20% blasts in the peripheral blood or bone marrow according to the IWG-MRT criteria [30]); (iii) death for any cause (overall survival).