In all these circumstances, interpretation of the experimental results is complicated by the time required for ras induction, and the inevitable cause-andeffect problems of the transformed state. Indeed, induction of ras overexpression causes rapid (within 60 min), selective alterations in the activity of cellular genes, including a sis-related sequence that may encode a growth factor6. As Owen and Ostrowski correctly note.," the time window in which ras action is monitored is important because the cellular milieu in which this gene operates is progressively altered". The generic problems of inducible constructions have therefore encouraged novel experimental designs that circumvent them.