Maya V. Georgieva et al. ; Protein tagging is widely used in approaches ranging from affinity purification to fluorescence-based detection in live cells. However, an intrinsic limitation of tagging is that the native function of the protein may be compromised or even abolished by the presence of the tag. Here we describe and characterize a set of small, innocuous protein tags (inntags) that we anticipate will find application in a variety of biological techniques. © 2015 Nature America, Inc. ; This work was funded by the Ministry of Economy and Competitiveness of Spain (INNPACTO IPT-010000-2010-19), Consolider-Ingenio 2010 (CSD2007-15), the Instituto Nacional de Bioinformática (INB) and the European Union (FEDER) and received financial support from Antibody BCN and Immunostep. M.O. acknowledges support from Institució Catalana de Recerca i Estudis Avançats ; Peer Reviewed