“…Immortalizing genes-SV40T [40], hTERT [42], or their combination (SV40T + hTERT [38,39,106])-are flanked by two homodromous LoxP sites [107,108] or two flippase recognition target (FRT) sites [109][110][111][112][113] and are delivered by a retroviral vector [107,108,110,111,114,115], CRISPR/Cas9 system [54], or by the PiggyBac transposon system (transposonmediated vector pMPH86) [55,112,113], which is one of the two most widely used transposon systems (PiggyBac, Sleeping Beauty) [56,116,117] and exhibits higher transposition activity than Sleeping Beauty in cultured human cells [117].…”