KLRK1 and carcinoma: Comparing NKG2D counterligand expression by primary breast duct epithelial cells with that of MDA-MB-231 carcinoma cells, we found that primary epithelial cells of either a luminal or basal phenotype lacked detectable expression of MICA/B or ULBP isoform 1, 2, 3, 5, or 6 (Fig. 6b).