A number of cyto,genetic and molecular analyses have revealed very frequent and extensive losses of regions of chromosome 10 in human glioblastomas. Our recent studies have demonstrated that the transfer of a chromosome 10 into human glioblastoma cells resulted in suppression of their transformed and tumorigenic phenotype. To localize the suppressive region further, we isolated and characterized certain hybrid cells that had undergone chromosomal rearrangements t o yield hybrid cells retaining only various regions of the inserted chromosome 10. One series of subclones showed the loss of the majority of the long arm of chromosome I0 (I Oq2 I -I Oqter) and regained the ability t o grow under anchorage-independent conditions, but the cells still failed t o exhibit significant tumorigenicity in nude mice. Another set of subclones exhibited major deletions of large segments of the long arm of chromosome I0 (I Oq2 I -q23; I Oq26-qter), yet retained certain distal alleles associated with I Oq24 to IOq26. These subclones were identical in their biological characteristics t o the hybrids containing an intact chromosome 10, exhibiting no growth in soft agarose or in nude mice. These results implicate the presence of two independent phenotypically suppressive regions on chromosome I0 (I Opter-q I I and I Oq24-q26) that are involved in glioma progression. Genes Chromosom Cancer 12:255-261 (1995).