Recognition of MHC and MHC-like molecules by both natural killer (NK) and T cell receptors (TCR) reveals remarkable degeneracy. The interaction of the NKG2D NK receptor with several MHC I-like ligands has now been analyzed thermodynamically by McFarland and Strong, who suggest that a ''rigid adaptation'' mechanism governs such crossreactivity. This contrasts with ''induced fit'' that accounts for TCR adaptation to multiple MHCp ligands.