This study is about testing a new drug called INCB057643 for people with a type of blood cancer called myelofibrosis (MF) and other similar diseases. Myelofibrosis is a condition where your bone marrow, which makes blood cells, doesn't work properly. Some participants will take INCB057643 alone, and others will take it with another medicine, ruxolitinib.

To join, you need to be at least 18 years old and have tried other treatments without success. You also shouldn't have any other treatment options that might help you, according to your doctor. You need to have a life expectancy of at least 24 weeks and agree to take precautions to avoid pregnancy.

• Study Length: Participants will be involved for a period based on the response to the treatment.
• Treatment: Some will get INCB057643 alone, others with ruxolitinib.
• Risks: Possible side effects and need to avoid certain medications.

If you have received another drug called a BET inhibitor before, you cannot join. Also, you must not have certain medical issues, like heart problems or active infections.