The U251 glioma were harvested, fixed in formalin, and serial sections were stained with anti-Ki67 (Vector Laboratories) and anti-geminin (Santa-Cruz) in the Research Pathology Shared Resource. For each tumor, at least 2 fields from each of 2 sections were photographed, each field representing about 1000 cells; 2–4 individual tumors were scored at each time point. The number of cells positive for geminin was expressed as a percentage of those positive for Ki67. Geminin is an established marker of S/G2 cells and readily amenable to analysis of cell cycle distribution in solid tumors, particularly as normal tissues are rarely proliferative. We have previously demonstrated that the use of geminin is best when combined with the Ki67 index which thereby corrects for differential proliferative index in different regions of a tumor 22 (link).