An important use for stem cells is the creation of cellular models of human development and disease, critical for uncovering the molecular roots of illness and testing new drugs. However, a major limitation in achieving these goals is the difficulty in manipulating human stem cells. We have developed a method of genetically modifying mouse embryonic stem cells that is more efficient than traditional methods. During the second year of this project, we took advantage of new methods using the CRISPR/Cas9 system to develop novel approaches to modifying human embryonic stem cells. We have also created reversible gene trap mutations in a diversity of human embryonic stem cell genes that can be used to better harness human embryonic stem cells for modeling, or even treating, human diseases.