The Y-box binding protein 1 (YB-1) is a DNA and RNA binding protein that performs numerous functions both in the cytoplasm and in the nucleus. Its nuclear localization has been observed at the G1/S cell cycle boundary [1], in stress conditions of various types [2-5], and under adenovirus infection [6]. The sequence of YB-1 contains a nuclear localization signal (NLS) [7] which is responsible for its nuclear translocation. The YB-1 NLS is recognized by transportin 1 [8] and classified with signals of the PY-NLS type characterized by the presence of an N-terminal positively charged or hydrophobic cluster, the residue R, and the C-terminal dipeptide PY. For better understanding of NLS functioning, we generated a few forms of YB-1 with mutations in its NLS region, where major structural elements of NLS were modified. We compared subcellular localization of the WT YB-1 with that of its mutant forms both in normal growth conditions and with stimulation of its translocation to the nucleus. Methods: The mutations were performed by site-directed mutagenesis. Transfection of eukaryotic cells was made according to the manufacturer’s recommendations. Results: The comparison of subcellular localization of WT YB-1 with that of its mutant forms showed that only the removal of the entire NLS resulted in the loss of the nuclear translocation ability of YB-1. The removal or replacement of separate structural elements of the NLS did not prevent YB-1 translocation to the nucleus either in normal growth conditions or under stimulated translocation. Conclusions: There is increasing evidence in the literature that some PY-NLS found in various proteins interact with transportin 1. Besides, transportin 1 can recognize PY-NLS lacking the dipeptide PY [9-12]. The YB-1 NLS has a highly charged N-terminal cluster which presumably makes the greatest contribution to the interaction with transportin 1. Funding: this work was supported by the RFFI (#18-34-00359\18) REFERENCES: 1. Jurchott, K., et al. (2003) J. Biol. Chem., 278, 27988–27996 2. Stein, U., et al. (2001) J. Biol. Chem., 276, 28562–28569 3. Das, S., et al. (2007) J. Biol. Chem., 282, 28474–28484. 4. Koike, K., et al. (1997) FEBS Lett., 417, 390–394. 5. Fujita, T., et al. (2005) Clin. Cancer Res., 11, 8837–8844. 6. Holm, P. S., et al. (2002) J. Biol. Chem., 277, 10427–10434. 7. Bader, A. G., et al. (2005) Mol. Cell. Biol., 25, 2095–2106. 8. Chook, Y.M., et al. (2011) Biochim. Biophys. Acta 1813, 1593–1606. 9. Soniat, M., et al. (2015) Biochem. J. 468, 353–362. 10. Twyffels, L., et al. (2014) FEBS Lett. 588, 1857–1868. 11. Soniat, M et al. (2016) Structure 24, 1802–1809.