by use of consensus sequences;McMahon et al., 2018;Uchanski et al., 2019). used in cell biology in place of antibodies. We explain how Affimers and nanobodies are generated, used and their advantages for super-resolution imaging. == Introduction == Antibodies have been widely used by cell biologists to localise proteins within cells using light microscopy since the development of monoclonal technology in the 1970s (Kohler and Milstein, 1975). However, since IACS-8968 S-enantiomer the development of super-resolution imaging approaches in the past few years, it has become clear that antibodies have several limitations for super-resolution imaging (Carrington et al., 2019;de Beer and Giepmans, 2020;Pleiner et al., 2017;Tiede et al., 2017). Specifically, in immunolabelling applications, their large size (150 kDa and 15 nm in length for IgG) often hinders effective penetration into tissues and densely packed subcellular structures (Maidorn et al., 2016). Moreover, the large size and flexibility of antibodies places the fluorophore at some randomised or unknown distance from the target proteins, limiting accuracy and precision in super-resolution imaging techniques (Frh et al., 2021). Several smaller probes have now been developed to overcome the problems of using large antibodies in imaging applications, including Affimers and nanobodies. Affimer reagents are small non-antibody binding proteins, with a molecular mass (12 kDa) 10 times smaller than antibodies (Tiede et al., 2017,2014), and they are less than 4 nm in length (Fig. 1A). Nanobodies are similarly sized (1214 kDa and less than 4 nm in length) antigen-binding proteins derived from the variable antigen-binding domains of heavy chain only antibodies, which are common in a few animal species, such as camelids and sharks (where they are termed variable new antigen receptors; vNARs) (Dooley and Flajnik, 2006;Dooley et al., 2003;Feige et al., 2014;Feng et al., 2019;Greenberg et al., 1993;Hamers-Casterman et al., 1993;Pleiner et al., 2015). Affimers and nanobodies are a little smaller than GFP (4.7 nm long, 27 IACS-8968 S-enantiomer kDa), SNAP (3.7 nm long, 20 kDa) and Halo (5 nm long, 33 kDa) tags. Affimers and nanobodies can thus improve one or more of the potential limitations of antibodies. Their small size enables better penetration of tissues and dense cytoskeletal structures, and places fluorophores much closer to the target protein being imaged in super-resolution microscopy, thereby improving resolution (Tiede et al., 2017). == Fig. 1. == Generation of Affimers and nanobodies.(A) Affimers are isolated by screening a phagemid library with a biotinylated protein of interest (POI) on a surface via streptavidin beads. For screening, the biotinylated target protein is bound IACS-8968 S-enantiomer to plastic-adsorbed streptavidin (first panning round), magnetic bead-conjugated streptavidin (second panning round) and plastic-adsorbed neutravidin (third panning round) (Tang et al., 2017;Tiede et al., 2014). In each panning round, after phage has bound to the immobilised target, the binding FLJ20353 matrix is extensively washed, then bound phage is eluted. To amplify binders for subsequent panning rounds, eluted phage is used to infect ER2738E. colicells, which are then co-infected with M13K07 helper phage to provide the additional proteins needed for phage replication (Vieira and Messing, 1987). The use of different biotin-binding matrices in the screening process acts to reduce carry-through of phage by non-specific interactions. To further enhance specificity, for example to create isoform-specific binders, rounds of negative selection can be added to the procedure as required (Tang et al., 2017). The phagemid DNA of high-affinity clones is sequenced to identify unique binders, which are then subcloned into a bacterial expression vector for Affimer production. (B) Nanobodies from camelids (or vNARs.