PTPN22 and Crohn disease: We also note the further complexity that a disease allele associated with susceptibility to one or more autoimmune disease can actually be protective for a different autoimmune disease, which is the case for the human R620W polymorphism in human LYP, where the rare 620W variant is associated with protection from Crohn disease, but with susceptibility for most autoimmune diseases where association has been discovered (11).