Internalins are surface proteins located on Listeria monocytogenes. Internalins exist in two known forms, InlA and InlB. Internalins are used by the bacteria to invade mammalian cells via cadherins transmembrane proteins. The precise role of the Internalin proteins and their invasiveness in vivo is not entirely understood. However, in cultured cells, InlA is needed to facilitate Listeria entry into human epithelial cells. While InlB is required for Listeria internalisation in several other cell types, including hepatocytes, fibroblasts, and epithelioid cells.

Internalin Listeria monocytogenes Recombinant produced in E.Coli is a single, non-glycosylated polypeptide chain containing 374 amino acids and having a molecular mass of 42.7kDa. The Internalin-A is fused with an 8 amino acid His tag at C-terminus.