CD19 and acute lymphoblastic leukemia: Based on the stages of differentiation of normal B cell progenitors, BCP-ALL is classified into pro-B ALL (CD19+ CD10− cyμ− Igκ− Igλ−), common-B ALL (CD19+ CD10+ cyμ− Igκ− Igλ−) and pre-B ALL (CD19+ CD10+/− cyμ+ Igκ− Igλ−) [1].