NLS (PKKKRKV) TFA is the TFA form of NLS (PKKKRKV). NLS (PKKKRKV) is a nuclear localization signal (NLS) derived from the simian virus 40 large tumor antigen (SV40 large T antigen), that mediates binding of the karyophilic protein to importin α. NLS (PKKKRKV) can function as a method to enhance nuclear entry in the field of gene transfer research.