MPL and Thrombocytosis: Using pairwise comparison, Zeng et al (21) identified that the frequency of the thrombopoietin receptor (TPOR) C allele at position 550 is significantly higher in subjects with high PLT counts, and that thrombocytosis is associated with a C to A transversion at position 550 in the 5′-promoter region of TPOR (21).