Recent technological developments have ushered in a new era for the medical research field based on our ability to generate stem cells (called induced pluripotent stem cells or iPSCs) out of adult patient cells, such as blood or skin fibroblasts. There are two important benefits of this technology relevant for research into Prader Willi Syndrome (PWS): 1) the newly generated stem cells retain all of the genetic information from the donor; and 2) these stem cells can be made to differentiate into any cell types, including neural cells. For neurological disorders with complex genetics, these two properties are particularly salient. We propose to generate iPSCs from four PWS patients with type I and type II deletions of paternally expressed genes in the 15q11.2-13 region. We will then differentiate these cells into human cortical neurons to investigate cellular and molecular mechanisms that contribute to disturbances in brain development that lead to PWS. Studies such as these are a much-needed complementary approach to animal models and patient studies. Importantly, we will be investigating disease-relevant human cells, with the same genetic disruption in the 15q11.2-q13 region as patients, in a controlled cellular model that is amenable to targeted manipulation of specific genes and processes. Results of this work could enable improved diagnostic classification and lay the groundwork for future drug screens to identify new therapeutic compounds.