Pluripotent stem cells can be induced from somatic cells, providing an unlimited cell resource, with potential for studying disease and use in regenerative medicine. However, genetic manipulation and technically challenging strategies such as nuclear transfer used in reprogramming limit their clinical applications. Here, we show that pluripotent stem cells can be generated from mouse somatic cells at a frequency up to 0.2% using a combination of seven small-molecule compounds. The chemically induced pluripotent stem cells resemble embryonic stem cells in terms of their gene expression profiles, epigenetic status, and potential for differentiation and germline transmission. By using small molecules, exogenous "master genes" are dispensable for cell fate reprogramming. This chemical reprogramming strategy has potential use in generating functional desirable cell types for clinical applications.

Somatic cells can be reprogrammed into pluripotent stem cells (PSCs) by using pure chemicals, providing a different paradigm to study somatic reprogramming. However, the cell fate dynamics and molecular events that occur during the chemical reprogramming process remain unclear. We now show that the chemical reprogramming process requires the early formation of extra-embryonic endoderm (XEN)-like cells and a late transition from XEN-like cells to chemically-induced (Ci)PSCs, a unique route that fundamentally differs from the pathway of transcription factor-induced reprogramming. Moreover, precise manipulation of the cell fate transition in a step-wise manner through the XEN-like state allows us to identify small-molecule boosters and establish a robust chemical reprogramming system with a yield up to 1,000-fold greater than that of the previously reported protocol. These findings demonstrate that chemical reprogramming is a promising approach to manipulate cell fates.

The introduction of four transcription factors Oct4, Klf4, Sox2 and c-Myc by viral transduction can induce reprogramming of somatic cells into induced pluripotent stem cells (iPSCs), but the use of iPSCs is hindered by the use of viral delivery systems. Chemical-induced reprogramming offers a novel approach to generating iPSCs without any viral vector-based genetic modification. Previous reports showed that several small molecules could replace some of the reprogramming factors although at least two transcription factors, Oct4 and Klf4, are still required to generate iPSCs from mouse embryonic fibroblasts. Here, we identify a specific chemical combination, which is sufficient to permit reprogramming from mouse embryonic and adult fibroblasts in the presence of a single transcription factor, Oct4, within 20 days, replacing Sox2, Klf4 and c-Myc. The iPSCs generated using this treatment resembled mouse embryonic stem cells in terms of global gene expression profile, epigenetic status and pluripotency both in vitro and in vivo. We also found that 8 days of Oct4 induction was sufficient to enable Oct4-induced reprogramming in the presence of the small molecules, which suggests that reprogramming was initiated within the first 8 days and was independent of continuous exogenous Oct4 expression. These discoveries will aid in the future generation of iPSCs without genetic modification, as well as elucidating the molecular mechanisms that underlie the reprogramming process.