PAX5 and acute lymphoblastic leukemia: To study the role of PAX5‐JAK2 (Appendix Fig S1A) in B‐ALL development, we used ES cell targeting to generate a mouse model by inserting human cDNA sequences, starting with exon 4 and encoding the remaining PAX5‐JAK2 protein, into the mouse Pax5 locus to recapitulate the corresponding human rearrangement as closely as possible (Fig 1A and Appendix Fig S1B and C).