In a report of 35 cases of clinically suspected CNL in whom JAK2V617F screening was performed, three individuals harbored JAK2V617F mutations but only one with truly WHO-defined CNL (but lacking the prototypical CSF3R mutation)76, supporting the apparent mutual exclusivity of JAK2 and CSF3R mutations, yet allowing for JAK2 positivity in exceptional cases of CNL. This evidence concerns the gene JAK2 and chronic neutrophilic leukemia.