Consistent with the "two-step" model, Pax5+/- and the Sca1-ETV6-RUNX1 mice housed exclusively in a specific pathogen-free (SPF) environment never developed B-ALL despite harbouring the genetic predisposition; however, when these predisposed mice were moved to a conventional facility where they are exposure to infectious agents, 22% of Pax5+/- and 10% of Sca1-ETV6-RUNX1 mice developed B-ALL (3, 32). This evidence concerns the gene ETV6 and precursor B-cell acute lymphoblastic leukemia.