@exon9 Statistically speaking, patients with the CALR mutation tend to have fewer blood clots than patients with the JAK2 mutation. Practically speaking, that means that hematologists may start hydroxyurea later for CALR patients under 60 with no clot history than for JAK2 patients.

@exon9 I don't claim to be a huge expert on this, but here goes. CALR is a protein that is involved in signaling the production of blood components. With MPN's in general, the DNA that codes for the CALR protein has been changed in someway to cause the increase in platelet production. Think of the DNA and proteins as letters and words. If the letters are in the correct order it makes the word we know, but in the wrong order its meaning is changed - think of meat vs meet or plane vs plain or rain vs rane [one is correct spelling, the other not]; they sound the same but the meanings are not. There a number of great resources to explain this in much greater detail (MPN Research Foundation - https://mpnresearchfoundation.org/ , Blood Cancer United - https://llsorg.widen.net/view/pdf/juiq8xkd2b/en-booklet-mpn-detail-ps81.pdf , Voices of MPN - https://www.voicesofmpn.com/essential-thrombocythemia) as well as your doctor.

Meds to help control this are constantly evolving and the scientific community is making some really interesting strides in this area, from some of the things I have seen lately there is reason for optimism in the not to distant future.