Next generation sequencing analysis, retrospectively performed on stored samples, showed a germ line <i>CSF3R</i> mutation, <i>CEBPA</i> clonal hematopoiesis, but no <i>RUNX1</i> lesion.<h4>Conclusion</h4>We describe a case of HR-MDS, with deletion of chromosome 7 and acquisition of <i>RUNX1</i> mutation, developing after CAR T-cell therapy in a patient with clonal hematopoiesis (CH). Here, RUNX1 is linked to myelodysplastic syndrome.