NIPA1 and Prader-Willi syndrome: The cardinal disease associations for the four contiguous genes in the 15q11.2 BP1-BP2 region are: NIPA1—Spastic Paraplegia 6; NIPA2—Angelman syndrome and Prader-Willi syndrome; CYFIP1—fragile X syndrome and autism; and TUBGCP5—Prader-Willi syndrome.