Murine radiation-induced acute myeloid leukaemia (AML) is characterized by loss of one copy of chromosome 2. Previously, we positioned the critical haematopoietic-specific transcription factor PU. 1 within a minimally deleted region. We now report a high frequency (> 65%) of missense mutation at codon 235 in the DNA-binding Ets domain of PU. 1 in murine AML. Earlier studies, outside the context of malignancy, determined that conversion of arginine 235 (R235) to any other amino-acid residue leads to ablation of DNA-binding function and loss of expression of downstream targets. We show that mutation of R235 does not lead to protein loss, and occurs specifically in those AMLs showing loss of one copy of PU. 1 (P= 0.001, Fisher's exact test). PU. 1 mutations were not found in the coding region, UTRs or promoter of human therapy-related AMLs. Potentially regulatory elements upstream of PU. 1 were located but no mutations found. In conclusion, we have identified the cause of murine radiation-induced AML and have shown that loss of one copy of PU. 1, as a consequence of flanking radiation-sensitive fragile domains on chromosome 2, and subsequent R235 conversion are highly specific to this mouse model. Such a mechanism does not operate, or is extremely rare, in human AML.