RUNX1 and leukemia: However, chimeric mice expressing CBFβ-SMMHC with deletion of the HABD (CBFβ-SMMHC-ΔHABD) only partially rescued the hematopoiesis phenotype, and unexpectedly accelerated leukemia development caused by CBFβ-SMMHC, challenging the importance of RUNX1 for CBFβ-SMMHC–mediated leukemogenesis.