APP and Alzheimer disease: To address the role of the supernumerary copy of the APP gene in AD-like neuropathology in DS, we deleted exon 3 of one of the APP alleles (Figure 1A) in a previously characterized footprint-free DS iPSC line, clone C11DS (Briggs et al., 2013) and this was confirmed by genomic PCR (Figure 1B) and Southern blotting (Figure 1C).