POU5F1 and neoplasm: These cells, isolated from one NFPT and one GH-secreting tumor, were able to form floating spheres when grown in stem cell-permissive medium (DMEM/F12, B27 supplement, bFGF, and EGF) (Figure 1), as described for normal pituitary stem cells (45); expressed stem cell markers (OCT4, CD133 NOTCH4 and Nestin) (Table 2) and pituitary-specific markers (PIT1 and <GSU); and could be subcloned in culture (self-renewal assays) and differentiated into pituitary hormone-producing cells.