This is a single-center, open-label, investigator-initiated phase 1 study designed to evaluate the safety, pharmacokinetics (PK), and preliminary efficacy of the menin inhibitor BY002 in patients with relapsed or refractory acute leukemia. Eligible subjects include adult patients (≥18 years) with AML, ALL, or MPAL, excluding APL, who carry KMT2A rearrangement or NPM1 mutation and have no better treatment options. The study will be conducted in a dose-escalation design (3+3) with an accelerated titration at the starting dose, followed by expansion at the recommended dose. BY002 is administered orally in 28-day cycles until disease progression, unacceptable toxicity, HSCT, withdrawal, or death. The primary objectives are to determine the incidence of dose-limiting toxicities (DLTs) and serious adverse events (SAEs), and to define the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D). Secondary objectives include characterization of PK parameters, evaluation of safety (AEs, laboratory tests, vital signs, ECG), and assessment of efficacy endpoints such as complete remission (CR), composite remission (CRc), overall response rate (ORR), duration of response (DOR), event-free survival (EFS), relapse-free survival (RFS), overall survival (OS), and cumulative incidence of relapse (CIR). Exploratory objectives include analysis of pharmacodynamic biomarkers (e.g., HOXA9, MEIS1, CD11b) and correlation of baseline genetic mutations (e.g., NPM1, KMT2A, FLT3, TP53, NUP98) with clinical outcomes.