Down Syndrome (DS) is the most abundant genetic form of mental retardation. It is caused by the triplication of partial or complete human chromosome 21 (HSA21). The molecular mechanisms causing it are not fully understood. Previous studies identified "Down syndrome Critical Region" (DSCR) genes that are essential or sufficient for the development of DS. However, these studies are largely inconclusive, due, in part, to the reliance on a small number of epidemiological cases. Amyloid precursor protein (APP) resides on HSA21 and is triplicated in DS. APP plays a role in developmental and post-natal neurogenesis, but is not thought to be part of the DSCR. The role of APP overdose in cortical malformation and cognitive impairments in DS is unknown. Mutations in APP cause familial Alzheimer's disease (FAD). However, whether APP overdose is sufficient for the development of Alzheimer's disease (AD) in DS is not fully understood. Here, we addressed the role of APP overdose in neuronal development and AD pathology. Using CRISPR/Cas9 gene editing, we eliminated one copy of APP from Down Syndrome-derived induced iPSCs DS APP(+/+/-) and examined the effect on neurogenesis, AD-related pathology and the expression levels of genes on HSA21 that are implicated in DS, neurodegeneration and inflammation.