Rearrangement of immunoglobulin chain genes is a hallmark of mammalian B cell development and differential expression of immunoglobulin chains has aided in the diagnosis and prognostication of patients with ALL. Matsu et al. describe two novel precursor B-cell lines with shift of expression of immunoglobulin phenotypes at diagnosis and relapse. The impact of this observation on diagnosis and the elucidation of the biology of the neoplastic cells will be discussed.