The integration of molecular findings with BM morphology and PB counts remains the cornerstone of diagnosis, including the BCR-ABL1 translocation in chronic myeloid leukemia (CML), the MPN-associated mutations JAK2 V617F, JAK2 exon 12, MPL 515L/K, and calreticulin (CALR) for the classical BCR::ABL1-negative MPN subtypes, and the presence of driver mutations in the colony-stimulating factor 3 receptor (CSF3R) in chronic neutrophilic leukemia. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.