Here we explore the clonal evolution of a form of childhood precursor-B cell acute lymphoblastic leukemia that is characterized by a chromosomal translocation generating a TEL-AML1 fusion gene.     ///    "By studying a monochorionic twin pair, one preleukemic and one with frank leukemia, we establish the lineal relationship between these ""cancer-propagating"" cells and the preleukemic cell in which the TEL-AML1 fusion first arises or has functional impact. "