However, the prognostic implications of improvements, or worsening, in bone marrow fibrosis or JAK2 V617F allele burden are unclear; given the prognostic significance of spleen size reduction with ruxolitinib treatment25 and the benefit that symptom improvement provides to patients, monitoring response to treatment in clinical practice by evaluating spleen size and symptoms, rather than with fibrotic score or allele burden, provides the best assessment of treatment benefit. This evidence concerns the gene JAK2 and primary myelofibrosis.