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Mutations in splicing factors are recurrent somatic alterations identified in myelodysplastic syndromes (MDS) and they frequently coincide with mutations in epigenetic factors. About 25% of patients present concurrent mutations in such pathways, suggesting a cooperative role in the pathogenesis of MDS. We focused on the splicing factor U2AF1 involved in the recognition of the 3′ splice site during pre-mRNA splicing. Using a CRISPR/Cas9 system, we created heterozygous mice with a carboxy-terminal truncated U2af1 allele (U2af1^mut/+), studied the <i>U2af1</i>^mut/+ hematopoietic system, and did not observe any gross differences in both young (12–13 weeks) and old (23 months) <i>U2af1</i>^mut/+ mice, except for a reduction in size of approximately 20%. However, hematopoietic stem/progenitor cells lacked reconstitution capacity in transplantation assays and displayed an aberrant RNA splicing by RNA sequencing. We also evaluated <i>U2af1</i>^mut/+ in conjunction with Tet2-deficiency. Novel double mutant <i>U2af1</i>^mut/+<i>Tet2</i>^−/− mice showed increased monogranulocytic precursors. Hematopoietic stem/progenitor cells were also enhanced and presented functional and transcriptomic alterations. Nonetheless, <i>U2af1</i>^mut/+<i>Tet2</i>^−/− mice did not succumb to MDS disease over a 6-month observation period. Collectively, our data suggest that cooperation between mutant U2af1 and Tet2 loss is not sufficient for MDS initiation in mice.