The apolipoprotein E receptor 2 (apoER2) gene consists of a mosaic of exons, which may have been assembled by "exon shuffling." Analysis of apoER2 transcripts in several species reveals a lost repeat in the ligand-binding domain of primate apoER2. A pseudo-exon found in the primate apoER2 genes corresponds to the lost repeat but contains a crucial deletion that leads to a translational frameshift. The pseudo-exon sequence in primary transcripts of the human apoER2 gene is shown to be abolished by exon skipping due to two nucleotide substitutions at the 5'-splice donor adjacent to the pseudo-exon. These data suggest the occurrence of exon loss in the evolution of the primate apoER2 gene.