Nevertheless, when crossed to additional leukemia predisposing backgrounds (e.g. Ebf1 or Ikzf1 [35], Stat5b [36] or BCR-ABLp190 [37]) or challenged through external stressors (e.g. infection exposure [20] or antibiotics [38]) Pax5± mice develop an aggressive BCP-ALL that closely resembles the human disease. This evidence concerns the gene BCR and infection.