Mutations in RNA splicing factor genes are common in patients with myelodysplastic neoplasms (MDS) and secondary acute myeloid leukemia (AML) [1][2][3].Cells bearing these mutations have been shown to be preferentially sensitive to pharmacologic modulation of splicing compared to wild-type (WT) cells [4,5].Protein arginine methyltransferase 5 (PRMT5) is an arginine methyltransferase which utilizes the methyl donor S-adenosylmethionine (SAM) to symmetrically methylate arginine residues on a variety of proteins [6,7].PRMT5-dependent symmetric di-methyl arginine (SDMA) mark is essential for spliceosome assembly and normal splicing [8].Several preclinical studies have demonstrated that PRMT5 inhibition results in synthetic lethality with splicing factor mutant leukemia cells [1,8].Here we describe the discovery and characterization of PRT543, an oral potent, selective inhibitor of PRMT5, in preclinical studies as well as a phase I dose-escalation/expansion trial in patients with relapsed/refractory (R/R) myeloid neoplasms (NCT03886831).This was a multicenter, open-label, sequential cohort, doseescalation/expansion phase I study of PRT543 monotherapy in patients with advanced solid and hematologic malignancies.Only data from patients with AML, MDS, and MDS/MPN (myeloproliferative neoplasm) overlap are reported here.Data from the doseescalation part leading to the identification of the recommended phase 2 dose (RP2D) of PRT543 35 mg daily 5x/week have been presented previously [9,10].Full details on study design are in Supplementary Methods.The primary endpoint of the dose escalation phase was to characterize dose-limiting toxicities (DLTs) and determine the RP2D of PRT543 monotherapy.In the dose expansion phase, the primary endpoint was best overall response rate (ORR) during first three response assessments.ORR was