Notably, the loss of heterozygosity in Pax5Jak2/+ B‐ALL tumors occurred by acquired uniparental disomy through copying the Jak2 cDNA insertion of the Pax5Jak2 allele by interchromosomal recombination into the wild‐type Pax5 allele. The gene discussed is JAK2; the disease is acute lymphoblastic leukemia.