Figure 1: Illustration demonstrating reversible cell immortalization. Reversible immortalization involves the introduction of immortalizing genes into primary cells, expanding the cells in culture, and ultimately efficiently removing the immortalizing genes. Immortalizing genes such as SV40Tag, and/or hTERT are expressed in transduced cells in the absence of Cre recombinase. After Cre/LoxP recombination, the intervening DNA segment between the two recombination targets is excised. The cells reverted to their preimmortalized state after Cre expression. LTR: Long Terminal Repeat; SV40Tag: Simian Virus 40 large T antigen; hTERT: Human Telomerase Reverse Transcriptase.