15q11.2 deletion syndrome is caused by a 300‐500 kb deletion of the BP1 to BP2 region in 15q11.2, which contains four non‐imprinted genes, TUBGCP2, NIPA1, NIPA2, and CYFIP1. The syndrome has a variety of phenotypes and is incompletely dominant. This evidence concerns the gene NIPA1 and Down syndrome.