An additional consideration when BCR sequencing from DNA, rather than RNA, is that in some B cells initial BCR rearrangements are defective and followed by rearrangement of the second BCR (IgH) allele,37 something seen commonly in B-ALL clones.38 B-ALLs arising from such B cells would thus mimic the presence of two independent leukemias when DNA is used to amplify/sequence BCRs. The gene discussed is BCR; the disease is acute lymphoblastic leukemia.