The goal of this project is to define the factors involved in choroid plexus epithelial cell (CPEC) development in mice, then to apply this knowledge to generate CPECs from mouse and human embryonic stem cells (ESCs) for clinical applications. The first two Aims used mice to examine a potential promoter and inhibitor, respectively, of CPEC fate, and the third Aim is to generate human CPECs in culture. Unexpectedly early success in CPEC derivation from human ESCs has allowed us to accelerate Aim 3 and the pursuit of translational goals this year. We further optimized our existing human CPEC derivation method and developed a second method (a combined suspension-adherent system) that may prove to be much more efficient. Several new GMP-compliant human ESC lines were approved and obtained. To facilitate the translational efforts, we made many new mouse ESC lines that were designed to fluoresce when CPECs are produced, and this was confirmed using the first of these lines. A crude CPEC purification strategy was also developed, and using this strategy, transplantation of partially-purified CPECs into mice was established in the lab this year. Remarkably, we found that transplanted mESC-derived CPECs, on their own, can integrate into endogenous choroid plexus with relatively high efficiency. This opens up several new and exciting therapeutic possibilities. To further enhance choroid plexus engraftment, a mouse CPEC ablation approach is currently being tested. A collaboration was initiated to profile all of the genes expressed by the purified mouse ESC-derived CPECs, and to compare this profile to those expressed by the choroid plexus in developing mice and humans. Industry partnerships and non-provisional patenting were also pursued to enhance the prospects for human CPEC applications in drug screening and treating patients with a wide range of neurodegenerative and other nervous system disorders.