EPAS1 and neoplasm: Tumor xenografts immunostained for HIF-2α followed the same trends as 3D spheroids in vitro, with MCF-7 SP and MDA-MB-231 SP displaying the greatest number of HIF-2α immunopositive cells/field (163 ± 0.25 and 175 ± 0.15, respectively) and ZR-75-1 SP having the lowest number of HIF-2α immunopositive cells/field (104 ± 0.1) (Figure 10).