 This is a Phase 1, open-label, dose-escalation study designed to evaluate the safety, tolerability, pharmacokinetics PK, pharmacodynamics PD and antineoplastic activity of BLU-285, administered orally PO, in adult patients with advanced systemic mastocytosis and other relapsed or refractory myeloid malignancies. The study consists of 2 parts, a dose-escalation part Part 1 and an expansion part Part 2. 