BioLock is a ready-to-use biotin blocking solution containing avidin. Avidin strongly binds to biotin and biotinylated proteins, but not to Strep-tag®II or Twin-Strep-tag®-fusion proteins. This way, unspecific binding of biotin/biotinylated proteins to Strep-Tactin® or Strep-Tactin®XT resins is prevented, ensuring an unimpaired binding capacity and a high purity of the target protein. Biotin and/or biotinylated proteins are present in cell lysates and cell culture media, especially from mammalian or insect cells. Thus, if Twin-Strep-tag® and Strep-tag®II fusion proteins from biotin-containing extracts or media are intended to be purified, biotin has to be masked by addition of BioLock to the sample prior to applying it to the resin. For masking biotin for western blot, we recommend using purified avidin.