CD19 and leukemia: In our previous work, we have challenged the hierarchical stem cell model for acute B lymphoblastic leukaemia (B-ALL) by demonstrating that phenotypically diverse blasts irrespective of the expression of the B-cell marker CD19 and the stem cell marker CD34 are able to re-establish the leukaemia in immunodeficient mice (le Viseur et al, 2008).