CALR and acquired polycythemia vera: As in PV, the SRSF2 mutation seems to be associated with a worse prognosis in ET, as well as the SF3B1, U2AF1 and TP53 mutations [51,52,78] The monitoring of CALR allele burden seems to be interesting to predict evolution [74] as for additional mutations, the study of the rate of the allele burden seems to be interesting for the leukemic transformation process [79].