CD19 and acute lymphoblastic leukemia: CD19 intron 2 transcripts are highly expressed at baseline in primary B-ALL cells and cell lines, however CD19 intron 10 transcripts are expressed at lower baseline levels such that smaller increments in intron 10 retained transcripts can be more easily detected, suggesting a potential prognostic role for CD19 intron 10 retention as a proxy for intron 2 retention and emergence of CD19 loss.