CALR and acquired polycythemia vera: In this review, we will focus strictly on the three most frequent Philadelphia chromosome (Ph) negative disease entities: PV, ET, and PMF, which are mostly associated with mutations in either the Janus kinase 2 (JAK2) gene, the myelproliferative leukemia virus oncogene (MPL), or the calreticulin (CALR) gene [2,3].