ULBP1 and astrocytoma (excluding glioblastoma): Constitutive levels of NKG2D ligands are low in most cells, thus, to examine the effect of U21 upon the surface expression of the NK ligands, we generated U373 astrocytoma cell lines individually stably expressing ULBP1, ULBP2, ULBP3, MICA, or MICB, using retrovirus-mediated gene transfer.