CLIP or the class II associated invariant chain peptide region of the invariant chain (Ii) has several important functions in the MHC class II antigen processing pathway. In specialized antigen presenting cells (APCs) such as macrophages, B cells and dendritic cells the MHC class II alpha-beta dimers assemble in the endoplasmic reticulum (ER) with a trimer of the nonpolymorphic invariant chain to form a nonameric complex which is transported through the Golgi apparatus to the trans Golgi network. In the trans Golgi network, signals to the invariant chain cytosolic tail target the complex to organelles in the endocytic pathway. Here the peptides derived from antigens, which are usually internalized into the endocytic pathway of antigenpresenting cells, are prevented from binding MHC class II molecules which are at the early stages of their bioynthesis. This prevention is mediated by the membrane proximal domain of Ii which is degraded upon arrival in the endocytic structures prior to peptide loading and these fragments are termed CLIP which consists of amino acids 81 to 104 of Ii.