On the cover: The 4-5 loop of Ly-49 receptors confers class I MHC allele specificity. Polymorphic residues in the site 2 interface are shown in ball-and-stick representation, attached to the main chain model of the Ly49A(red)/H-2D^d (blue) complex crystal structure. The invariant ₂-microglobulin is depicted in green. Ma et al. Reciprocal transfer of class I MHC allele specificity between activating Ly-49P and Ly-49W receptors by exchange of 4-5 loop residues. 5337.