The import of proteins into nucleus is targeted by nuclear localization signal (NLS) in the protein molecule. The proteins containing NLS bind the specific NLS-receptors, then cross nuclear pores and translocate into nucleus. The process of nuclear import requires the activation of cytosolic transport factors, nuclear pore complexes and the components of import machinery. When NLS is modified or masked it can not be recognized by components of the machinery, so the NLS-proteins are retained in the cytoplasm before the masks are released. Controlling the activities of transcription factors through modulating nuclear import of the proteins leads a new concept of regulation of gene expression and constitutes a regulatory level in cellular growth and differentiation.