The prognosis for most patients with astrocytic glioma is poor, and postoperative life expectancy has not significantly improved in the last decade despite advances in diagnosis, surgery, and adjuvant therapy. Progress has been made, however, in cataloging the genetic alterations that occur in these tumors. Studying the allelic changes using loss of heterozygosity analysis has proven to be a reliable and rapid way of identifying genetic alterations fundamental to the pathology of this disease. In this study, we used a series of fluorescent-labeled markers and a new horizontal ultrathin gel electrophoresis technology (HUGE; GeneSys Technologies, Inc.) to analyze loss of heterozygosity on 11p15 in a series of 24 matched normal/tumor glioma pairs that included both anaplastic astrocytomas and glioblastomas. These studies significantly narrowed the region harboring a putative 11p15.5 glioma-associated gene and further suggest that a second gene involved in the pathogenesis of brain tumors may exist, centromeric, in bands 11p15.5-p15.4.