PTPN22 and Alzheimer disease: Genetic predisposition for disease is evident in both dogs and humans [1, 8, 15, 16, 19, 24, 25], and genes implicated in increased susceptibility to human AD include the major histocompatibility complex (MHC) class II genes (haplotypes DR3-DQ2 and DR4-DQ8), cytotoxic T-lymphocyte-associated protein 4 (CTLA4), protein tyrosine-phosphatase non-receptor type 22 (PTPN22), MHC class II transactivator (CIITA) [19, 24, 26], and the autoimmune regulator (AIRE) gene, which causes an autoimmune polyglandular syndrome that includes AD [24].