The purpose of this study is to assess the real-world safety of fedratinib for the treatment of adult participants with primary myelofibrosis (PMF), post polycythemia vera myelofibrosis (post-PV MF), or post essential thrombocythemia myelofibrosis (post-ET MF) who were previously treated with ruxolitinib. Participants will represent the overall patient population with PMF, post-PV MF or post-ET MF who lost adequate response to and/or are intolerant to ruxolitinib. Inadequate response definitions will follow Ministry of Food and Drug Safety-approved label and reimbursement criteria of the Health Insurance Review & Assessment Service.

Inclusion Criteria:

• Participants 19 years of age or older
• Participants who will receive fedratinib according to the approved label
• For the first 2 years after marketing authorization, all participants who have received or are receiving fedratinib will also be registered
• Participants who signed the informed consent form

Exclusion Criteria:

• Participants who have been prescribed fedratinib for an indication not approved in Korea
• Participants who have been prescribed fedratinib at a dose not approved in Korea
• Participants for whom fedratinib is contraindicated as clarified in Korean prescribing information approved by the Ministry of Food and Drug Safety

Adult participants with primary myelofibrosis, post-polycythemia vera myelofibrosis, or post-essential thrombocythemia myelofibrosis who were previously treated with ruxolitinib