prostate tumors wIth Mill allelic loss. To determine whether Mill can indeed function as a suppressor of growth, we have introduced a steroid InducibleMill expression vector into the US7MGcell line, a glioblastoma

The Mxii protein functions in a regulatory network with members of the c-Myc family, In which c-Myc activates transcription and stimulates cell proliferation, and Mxii negatively regulates these actions. Inactiva tioa of the Mill gene could, therefore, inhibit differentiation and enhance proliferation in the presence of normal levels of c-Myc, and thus MXIJ is a potential tumor suppressor gene. We and others have previously mapped the MXIJ gene to the distal portion of chromosome lOq, a region that is rearranged or affected by allelic loss in many astrocytlc brain tumors. Using a newly described polymorphic CA microsatellite repeat In the third Mxli intron, we show that 7 of 11 informative glioblastomas demonstrated Mill allelic loss. Sequence analysis revealed no somatic mutations in any of the six Mill coding exons, similar to findings In prostate tumors wIth Mill allelic loss. To determine whether Mill can indeed function as a suppressor of growth, we have introduced a steroid InducibleMill expression vector into the US7MGcell line, a glioblastoma cell line lacking endogenous Mlii expression. Induction of MXIJ expres Mon resulted In a decreased growth rate and distinct morphological changes. Furthermore, cell cycle analysis demonstrated that induction of MXIJ results in accumulation of cells in the G2-M phase. Thus, these studies support the notion that MXIJ normally functions to suppress cell growth and suggest that loss of Mill function may play a role in human glioblastoma development.