**5. Future and challenges**

The original protocol to generate hiPSCs involves four transcriptional factors, but this method is not suitable because of its effect on genome integrity via the introduction of additional plasmids with exogenous genes. To make hiPSCs and their derivatives applicable for clinical uses, many improvements have been made to optimize the method for iPSC generation. The integration-free and chemical reprogramming protocols have been developed to minimize the risk of jeopardizing genome integrity [115, 116].

In general, the genetic nature of a disease, the molecular editing platform used, the delivery method, and the targeted cells and organs are all factors that influence the efficacy of treatment and determine the likelihood of clinical benefits [117]. The CRISPR/Cas9 molecular scissor system has been used to edit the genomes of a diverse array of mammalian cell types and organisms with high efficiency and precision. Determining and overcoming the actual frequency of off-target activities is challenging, yet critical to the application of the technology in gene therapy. CRISPR/Cas9 technology allows the study of complex genetic diseases, including human cancer, in which multiple mutations and chromosomal translocations are present in the genome [118, 119].

The potential application of hiPSC technology in cancer studies has been proposed, based on the idea of reprogramming cancer cells via hiPSC technology to cancer stem cell (CSC) state. CSCs are well-known as the origin of tumor development, the seeds for distant metastasis, and are critical in therapeutic resistance. Reprogramming the malignant cells back to their original state before the oncogenic transformation occurs [120], may provide tools for exploring the mechanisms of tumor initiation and progression *in vitro*, for studying the heterogeneity and origin of CSCs, and for producing cancer type-specific drug discovery. However, these reprogramming methods remain a challenge because of the cancer-specific epigenetic state and chromosomal aberrations of cancer cells.
