Telomeres are specialized structures of repetitive nucleotide sequences that cap the ends of human chromosomes. Their main purpose is to maintain genome stability and integrity, and to protect the cell from progressive DNA shortening during repeated division. Human enzyme telomerase complex maintains the length of telomere repeats.1 Despite the high levels of telomerase activity in cancer cells, telomere shortening can still occur. Some recent reports describe reduced telomere length in myelofibrosis (MF) regardless of hydroxycarbamide therapy, suggesting a possible prognostic relevance for this biomarker.2, 3, 4 As yet, no studies have investigated telomere dynamics following treatment with ruxolitinib, a JAK1/2 inhibitor approved for the treatment of intermediate-2 and high risk MF, primary or post-polycythemia vera (PV) and essential thrombocytemia (ET).5