Many molecular tools are now available to direct heterologous protein secretion in L. lactis and the list of heterologous proteins produced in this bacterium is regularly increased. The reports where cytoplasmic and secretion production can be compared mostly show that secretion allows better protein yields compared to intracellular Schematic presentation of the molecular tools and the cellular events that can affect the production yields of heterologous pro-tein in L. lactis Figure 4 Schematic presentation of the molecular tools and the cellular events that can affect the production yields of heterologous protein in L. lactis. Thicknesses of the arrows are proportional to the final production yields. All the host factors involved in the cellular events are not identified and or characterized yet. SP, signal peptide (encoded in pSEC constructions), +Nuc, fusion between the protein of interest and the stable Nuc protein. production; and allow a better understanding of the protein production and secretion process in L. lactis.