Genetic tools to target a given protein in different cellular compartments were developed using several reporter proteins [6, [11] [12] [13] (Table 1 ). The staphylococcal nuclease (Nuc) is a well-characterized secreted protein whose activity is readily detectable by petri plate assay and it has been used as a reporter protein for secretion studies in several Gram positive hosts [14] [15] [16] . In L. lactis, Nuc was used to develop protein targeting- [6] and SP screening-systems [1, 2] . Nuc was chosen to develop the pCYT and pSEC vectors for controlled production in L. lactis of cytoplasmic or secreted forms of a protein of interest, respectively ( Fig. 1 ) [5] . The pCYT and pSEC plasmids, where expression is controlled by a nisin inducible promoter, should be used in L. lactis NZ9000 (hereafter referred to as NZ) strain bearing a nisR,K chromosomal cassette, required for the nisin signal transduction [17] . In each case described below, protein sample concentration was adjusted to the cell density of the producing culture (for details see [18] ). At similar induction levels in lactococcal strains containing pCYT:Nuc and pSEC:Nuc vectors, the highest production yields were observed with the secreted Nuc form ( Table 2) . Similar results were obtained with constitutive nuc expression cassettes for cytoplasmic and secreted forms. Nuc was the first heterologous protein where highest protein yields were obtained with the secreted form.