NLS (PKKKRKV) is a peptide derived from the simian virus 40 large tumor antigen (SV40 large T antigen), as a method to enhance nuclear entry in the field of gene transfer research[1][2].
[1]. Zanta MA, et al. Gene delivery: A single nuclear localization signal peptide is sufficient to carry DNA to the cell nucleus. Proc Natl Acad Sci U S A. 1999 Jan 5;96(1):91-6. [2]. Cao S, et al. Enhanced effect of nuclear localization signal peptide during ultrasound-targeted microbubble destruction-mediated gene transfection. Mol Med Rep. 2017 Jul;16(1):565-572.