A key quality of stem cells is their ability to switch from a proliferative cell state in which they reproduce themselves to a differentiated cell state that ultimately allows them to carry out the functions of a fully mature cell. Most research on the nature of this switch has focused on the role of proteins that determine whether the genetic material—DNA—generates a copy of it itself in the form of messenger RNA, a process called transcription. In stem cells, such proteins—which are called transcription factors—activate the production of messenger RNAs encoding proteins that promote the proliferative and undifferentiated cell state. They also increase the production of messenger mRNAs that encode inhibitors of differentiation and cell proliferation. The level and profile of such transcription factors are altered in response to signals that trigger stem cells to differentiate. For example, transcription factors that promote the undifferentiated cell state are decreased in level and transcription factors that drive differentiation down a particular lineage are increased in level. While this transcription factor-centric view of stem cells explains some aspects of stem cell biology, it is, in of itself, insufficient to explain many of their behaviors, including both their ability to maintain the stem-like state and to differentiate. We hypothesize that a molecular pathway that complements transcription-base mechanisms in controlling stem cell maintenance vs. differentiation decisions is an RNA decay pathway called nonsense-mediated RNA decay (NMD). Messenger RNA decay is as important as transcription in determining the level of messenger RNA. Signals that trigger increased decay of a given messenger RNA leads to decreased levels of its encoded protein, while signals that trigger the opposite response increase the level of the encoded protein. Our project revolves around two main ideas. First, that NMD promotes the stem-like state by preferentially degrading messenger RNAs that encode differentiation-promoting proteins and proliferation inhibitor proteins. Second, that NMD must be downregulated in magnitude to allow stem cells to differentiate. During the progress period, we obtained substantial evidence for both of these hypotheses. With regard to the first hypothesis, we have used genome-wide approaches to identify hundreds of messenger RNAs that are regulated by NMD in both in vivo (in mice) and in vitro (in cell lines). To determine which of these messenger mRNAs are directly degraded by NMD, we have used a variety of approaches. This work has revealed that NMD preferentially degrades messenger RNAs encoding neural differentiation inhibitors and proliferation inhibitors in neural stem cells. In contrast, very few messenger RNAs encoding pro-stem cell proteins or pro-proliferation proteins are degraded by NMD. Together this provides support for our hypothesis that NMD promotes the stem-like state by shifting the proportion of messenger RNAs in a cell towards promoting an undifferentiated, proliferative cell state. During the progress period, we have obtained considerable evidence that this hypothesis not only applies to mouse stem cells but also human embryonic stem cells. With regard to the second hypothesis, we have found that many proteins that are directly involved in the NMD pathway are downregulated upon differentiation of stem and progenitor cells. Not only are NMD proteins reduced in level, but we find that the magnitude of NMD itself is reduced. We have used a variety of molecular techniques to determine whether this NMD downregulatory response has a role in neural differentiation and found that NMD downreglation is both necessary and sufficient for this event. Such experiments have also revealed particular messenger mRNAs degraded by NMD that are crucial for the NMD downregulatory response to promote neural differentiation. During the progress period, we obtained both experimental and genome-wide data that this applies to human embryonic stem cells. Our research has implications for intellectual disability cases in humans caused by mutations in an X-linked gene essential for NMD. Patients with mutations in this gene—UPF3B—not only have intellectual disability, but also schizophrenia, autism, and attention-deficit/hyperactivity disorder. Thus, the study of NMD may provide insight into a wide spectrum of cognitive and psychological disorders. We are currently in the process of generating and characterizing induced-pluripotent stem (iPS) cells from intellectual disability patients with mutations in the UPF3B gene towards this goal.