Somatic mutations in genes, such as V617F mutation in Janus kinase 2 (JAK2), JAK2V617F, JAK2 exon12, calreticulin (CALR) exon9, or MPL exon10, exhibit a high prevalence (almost 90%) among patients with MPN, and are the underlying etiology of these disorders (4–8). The gene discussed is CALR; the disease is myeloproliferative neoplasm.