We have established H-2Dd-transgenic (Tg) mice, in which H-2Dd expression can be extinguished by Cre recombinase-mediated deletion of an essential portion of the transgene (Tg). NK cells adapted to the expression of the H-2Dd Tg in H-2b mice and acquired reactivity to cells lacking H-2Dd, both in vivo and in vitro. H-2Dd-Tg mice crossed to mice harboring an Mx-Cre Tg resulted in mosaic H-2Dd expression. That abrogated NK cell reactivity to cells lacking Dd. In Dd single Tg mice it is the Ly49A+ NK cell subset that reacts to cells lacking Dd, because the inhibitory Ly49A receptor is no longer engaged by its Dd ligand. In contrast, Ly49A+ NK cells from Dd × MxCre double Tg mice were unable to react to Dd-negative cells. These Ly49A+ NK cells retained reactivity to target cells that were completely devoid of MHC class I molecules, suggesting that they were not anergic. Variegated Dd expression thus impacts specifically missing Dd but not globally missing class I reactivity by Ly49A + NK cells. We propose that the absence of Dd from some host cells results in the acquisition of only partial missing self-reactivity.