PTPN22 and Alzheimer disease: Candidate gene approaches have implicated many of the same genes identified in human AD with increased susceptibility to canine AD: CTLA4 in the PWD, Cocker Spaniel and Springer Spaniel [10, 14, 28]; PTPN22 in the Cocker Spaniel [10]; and the canine MHC (or dog leukocyte antigen, DLA) class II genes in the NSDTR, Bearded Collie and Standard Poodle [29–31].