In this study, we performed genetic sequencing of intron 12 of the JAK2 gene to identify the rs10974944 variant (C > G), in strong linkage disequilibrium with the 46/1 haplotype, in 100 patients with BCR::ABL1-negative myeloproliferative neoplasms (polycythemia vera: n = 39; essential thrombocythemia: n = 61) for whom clinical and laboratory information was available for clinical and laboratory characterization. The gene discussed is BCR; the disease is essential thrombocythemia.