To investigate the role of reduced PU.1 in AML, we utilised a radiation-induced murine AML cell line X18.1.1,18 characterised by hemizygous deletion of one Pu.1 allele, and two missense mutations in the remaining allele: an R235C substitution within the DNA-binding domain and a 30 amino acid deletion affecting the PEST domain (Figure 1a). This evidence concerns the gene SPI1 and acute myeloid leukemia.