A peptide-array analysis (Number 3) confirmed that the entire CD22 gene was well represented in the-lactamase-positive infectious phages, except two small regions of 7 residues each. to create such proteins involves fusing polypeptides into an end-to-end configuration. For example, the fusion of proteins with specific tagging domains is usually nowadays a standard tool for protein engineering. Indeed, numerous commercial kits allow the fusion of a protein of interest to an affinity polypeptide such as 6-Histidine tag, maltose-binding protein, and glutathione transferase [1]. Moreover, the fusion of a protein of interest to the green fluorescent protein (GFP) has become one of the most important tools used in contemporary bioscience, and O. Shimomura, M. Chalfie, and R. Y. Tsien were rewarded the Chemistry Nobel prize in 2008 for the initial discovery of GFP and development of important GFP-based applications. Another strategy to produce hybrid proteins, although much less commonly used, is to insert (or graft) a peptide/protein/fragment thereof at a permissive site of a carrier protein. For example, Collinet et al. [2] have successfully created bi- or trifunctional hybrid enzymes by inserting the UNC0638 dihydrofolate reductase (159 UNC0638 aa) and/or the TEM-1-lactamase (263 aa) into four different positions of phosphoglycerate kinase (415 aa). Betton et al. [3] have created hybrid proteins by inserting the TEM-1-lactamase at various sites into the MalE maltodextrin-binding protein, that is, positions 120, 133, and 303, or by fusing it to the carboxy terminus of MalE. Insertion at positions 133 and 303 or fusion at the C-terminus allows the production in the periplasm of hybrid proteins exhibiting both parental activities; indeed, the maltose binding and the penicillinase activity of these hybrid proteins is usually indistinguishable from that of, respectively, MalE and TEM-1 [3]. Interestingly, the two proteins with insertions displayed two additional properties compared to their C-terminal fusion counterpart: (i) they were more resistant to degradation UNC0638 by endogenous proteases and, (ii) even more remarkably, the TEM-1 moiety UNC0638 was stabilized against urea denaturation through the binding to MalE of maltose, that is, its natural ligand. This latter observation clearly demonstrates that TEM-1 is usually structurally dependent on MalE or in other words that an allosteric conversation occurs between the two proteins [1,3]. Thus, insertion of a polypeptide inside a UNC0638 scaffold protein can present some advantages compared to the more common end-to-end fusion. This strategy actually mimics that used by nature to generate protein diversity. Indeed, genetic rearrangements, such as the introduction of a sequence into an unrelated coding sequence, naturally occur in the genome [2]. The resulting proteins are composed of two or more domains, and the linear sequence of one domain name is interrupted by the insertion, forming discontinuous domains. This process has been shown for large natural proteins such as disulfide bond isomerase A (dsbA) [4], DNA polymerase [5], and pyruvate kinase [6]. A systematic survey of structural domains showed that ~28% of them are actually not continuous, clearly indicating that the sequence continuity of a domain is not required for correct folding and function [2,7]. In this review, we report the use of two class A-lactamases to create hybrid proteins (-lactamase hybrid proteins, BHPs) in which exogenous peptides/proteins/fragments thereof are inserted within-lactamase sequences. Such hybrid proteins can be designed for a range of applications C13orf1 such as (i) the creation of bifunctional hybrid proteins [811], (ii) the production, purification, and characterization of proteins otherwise difficult to express [10,12,13], (iii) the determination of the epitope of a specific antibody at the surface of an antigen [8,11], (iv) the generation of antibodies against specific antigens or fragments thereof [9,1315], and (v) the investigation of fundamental aspects of structure, stability, and function of proteins.