Tumor suppressor genes are a group of genes whose function is frequently lost through mutation cancer cells. We have identified Ikaros as a protein with characteristics of a novel T lineage specific tumor suppressor. With 100% penetrance, lack of Ikaros activity in mice results in leukemogenesis. We have shown, using a novel cell culture system, that genetic restoration of Ikaros to an Ikaros null T leukemia cell line results in cell cycle arrest and initiation of a T cell specific program of gene expression. We have also identified the cell cycle regulator, p27kip1, as a potential genetic target for transcriptional activation by Ikaros. Interestingly, decreased levels of p27kip1 gene expression have been linked to the mechanism of Bcr/Abl induced human leukemia, suggesting that decreased p27kip1 levels in Ikaros null T cells may hold the key to their remarkable propensity to transform. Ikaros may play a tumor suppressive role in human cells as well, since a high percentage of analyzed infant and childhood acute lymphocytic leukemias display defects in Ikaros gene expression. The goal of the experiments described in this proposal is to define mechanisms by which lack of Ikaros activity leads to leukemogenesis. We will do this by: 1) identifying Ikaros' functional/interaction domains essential to its role in growth control, 2) defining the mechanism by which Ikaros controls expression of the p27kip1 gene and 3) identifying Ikaros' functional/interaction domains required for its role in initiating a T cell specific program of gene expression. These studies will also include analyses of the role of Ikaros' association with Brg-1 and the Swi/Snf chromatin remodeling complexes in T cell growth control and induction of a T cell specific program of gene expression. The difficulty in defining mechanisms of Ikaros function has been due, at least in part, to lack of an easily manipulated experimental system. However, our unique cell culture system consisting of an Ikaros null T leukemia cell line that can be genetically altered using retroviral transduction is a potent tool to define mechanisms of Ikaros function. This system, in conjunction with the Ikaros null mice, makes us uniquely qualified to define the mechanistic role of this novel tumor suppressor gene.