Reprogramming highly depends on the efficient delivery and the suitable expression of certain factors into specific cell types, under particular culture conditions and within a period of time. Although direct reprogramming is a simple technique, it differs depending on the cell type, species and delivery method. It is rather a slow and vulnerable process that may be affected by several factors that hinder the efficiency, reproducibility and the quality of the resulting iPSCs. To date, the most popular donor somatic cells are fibroblasts, being used in more than 80% of all reprogramming experiments published (González et al., 2011). Yet, other cell types have been used in reprogramming such as human primary keratinocytes, cord blood CD133+ cells, and peripheral blood mononuclear cells (Aasen et al., 2008; Giorgetti et al., 2009; Su et al., 2016).
