Dangerous fusions: a path to cancer for arrested lymphoid progenitors.

B‐cell precursor acute lymphoblastic leukemia (BCP‐ALL) is a common malignancy associated with variable chromosomal translocations, leading to fusion proteins of unknown function. To investigate how such translocations contribute to the development of BCP‐ALL Smeenk (2017) generated mouse models for Pax5 fusion proteins. The results show that a PAX5 fusion is required for BCP‐ALL development by preventing B‐cell differentiation and retaining cells in an arrested progenitor stage. The occurrence of further genetic aberrations eventually results in oncogenic transformation and proliferation of the arrested cells, triggering the onset of leukemia.