Reelin is a 400 kD, secreted glycoprotein that belongs to the reelin family of proteins. It is a serine protease that degrades fibronectin and laminin. Neuronal secretion results in neuron migration and patterning. It also binds Apoer2 that contains exon19, resulting in enhanced memory and learning. Mature mouse reelin is 3435 amino acids (aa) in length. It contains one N-terminal reelin domain (aa 27-191), followed by eight EGF- like and 15 BNR interspersing sequences. There is a serine protease site between Phe1280 and Asp1286. There are at least three potential reelin isoforms. Two utilize an alternate start site at Met2581, with one of these also showing an additional two aa deletion (Val3429Ser3430). A third isoform shows a premature truncation after Leu3428. Over aa1221-1661 of the precursor, mouse reelin is 93% and 96% aa identical to human and rat reelin, respectively; over aa1-861 of the mature form, mouse reelin is 94% and 96% identical to human and rat reelin, respectively.