Chromosomal aberrations were examined in a breast cancer patient 11 years after cessation of cyclophosphamide methotraxate and 5-fluorouracil (CMF) therapy. This indicates that they may well be derived from a population of genetically damaged progenitor cells, from which abnormal and possibly malignant cell clones may have been generated, and a fraction of these randomly occurring chromosome rearrangements appear to be 'stable'.