In the event that a RUNX1 mutation carrier develops acute leukemia or MDS requiring allogeneic stem cell transplantation, genetic testing should be performed urgently on human leukocyte antigen (HLA)-matched related donors in whom the RUNX1 status is not previously known, and those who also carry the mutation should not be used as donors. The gene discussed is RUNX1; the disease is myelodysplastic syndrome.