MYC and cancer: Although efficient gene delivery systems using retroviral and/or lentiviral vectors have been used to introduce reprogramming factors into somatic cells, these viral vector systems remain controversial due to multiple copies of proviral genomic integration, which may cause both the reactivation of silenced exogenous oncogenes such as Klf4 and c-Myc and the alteration of genomic construction, thereby increasing the risk for malignant cancer transformation [60].