CPQ and neoplasm: The process of presenting tumor antigen peptides to the cell surface by MHC-I through the endogenous pathway can be mainly divided into four steps: 1) the endogenous proteins are degraded by the proteasome to form antigenic peptides; 2) the antigenic peptides are transported to the endoplasmic reticulum (ER) through the transporter associated with antigen presentation (TAP); 3) further trimming of antigenic peptides by ER aminopeptidase; 4) finally, these antigenic peptides are loaded by MHC-I molecules and then transported to the cell surface (Neefjes et al., 2011).