The basic structures of MHC class I and class II molecules are now wel
l established. Over the past twelve months structural data on MHC clas
s I molecules have provided details of the peptide binding groove for
a number of alleles and have elaborated the mechanisms that allow bind
ing of a range of peptides. Recent MHC class II structures have illust
rated the mode of peptide binding both in mature complexes and in the
MHC class II complex with a fragment of invariant chain (CLIP) during
maturation.