This multi-stage model is supported by the observation that chromosomal translocations/fusion genes (e.g., TEL-AML1) involved in producing ALL are detected at birth (prenatal event), and a postnatal event (e.g., TEL deletion) is required for disease manifestation.     ///    "Furthermore, we discuss use of an initiated animal model for ALL, namely transgenic mice with TEL-AML1 expression, for exploring mechanisms by which different classes of environmental exposures could be involved in inducing the postnatal step in ALL formation.. "