The initiation of ETV6/RUNX1 expression in HSCs and maintenance in their offspring in transgenic mice (ETV6-ETV6-RUNX1 × Sca1-Cre) leads to the development T-ALL (34.4%) and B-ALL (6.3%), when the mice are exposed to natural infections in a non-specific pathogen-free environment. The gene discussed is RUNX1; the disease is precursor B-cell acute lymphoblastic leukemia.