INCB000928 Administered as a Monotherapy or in Combination With Ruxolitinib in Participants With Anemia Due to Myeloproliferative Disorders

Inclusion Criteria: * Participants with MF who are transfusion-dependent or present with symptomatic anemia, defined as follows: 1. Anemia: An Hgb value \< 10 g/dL demonstrated during screening recorded on 3 separate occasions with at least 7 days between measurements (Note: RBC transfusion must be at least 2 weeks before the Hgb measurement during screening). 2. Transfusion-dependent: Participant has received at least 4 units of RBC transfusions during the 28 days immediately preceding Cycle 1 Day 1 OR has received an average of at least 4 units of RBC transfusions in the 8 weeks immediately preceding Cycle 1 Day 1, for an Hgb level of \< 8.5 g/dL, in the absence of bleeding or treatment-induced anemia. In addition, the most recent transfusion episode must have occurred in the 28 days before Cycle 1 Day 1. * ECOG performance status score of the following: 1. 0 or 1 for the dose-escalation stages. 2. 0, 1, or 2 for the dose-expansion stage. * Life expectancy is greater than 6 months * Agreement to avoid pregnancy or fathering children. * Ineligible to receive or have not responded to available therapies for anemia such as ESAs. * For TGA: * Participants previously treated with JAK inhibitors for at least 12 weeks. * Participants with intermediate-2 or high DIPSS MF according to IWG-MRT criteria. * For TGB: * Participants must have been on a therapeutic and stable regimen of ruxolitinib for at least 12 consecutive weeks immediately preceding the first dose of study treatment. * Participants with intermediate-1, intermediate-2, or high DIPSS MF according to IWG-MRT criteria. * For TGC: * Participants must be JAK inhibitor treatment naive (no prior treatment with any JAK inhibitor) and have an indication for initiation of ruxolitinib treatment. * Participants with intermediate-1, intermediate-2, or high DIPSS MF according to IWG-MRT criteria. Exclusion Criteria: * Undergone any prior allogenic or autologous stem cell transplantation or a candidate for such transplantation. * Any prior chemotherapy, immunomodulatory drug therapy, immunosuppressive therapy, biological therapy, endocrine therapy, targeted therapy, antibody or hypomethylating agent to treat the participant's disease, with the exception of ruxolitinib for TGB only, within 5 half-lives or 28 days (whichever is shorter) before the first dose of study treatment. * Laboratory Values outside of protocol defined range at screening.