The invention provides the components of in vivo and in vitro systems and methods which use them to study the effects of altered expression of a gene activity, such as the human akt, bcl-2, eIF4E or PTEN activities, on the descendants of stem cells that have been engineered to give rise to hematopoietic tumorigenic or tumor cells, such as lymphomas, with a high frequency. The present invention provides vectors, cells and mammals, and methods which in part depend on such products, useful for understanding tumorigenesis and its treatments, and in particular, for identifying and studying inhibitors and activators associated with tumor cell growth and growth inhibition, cell death through apoptotic pathways, and changes in apoptotic pathway components that affect drug sensitivity and resistance in tumorigenic cells. Methods for identifying molecular targets for drug screening, identifying interacting gene activities, for identifying therapeutic treatments and for identifying candidates for new therapeutic treatments are provided.