Although IL-25 (originally termed IL-17E) shares structural similarity with other IL-17 family members it acts in an opposing manner to IL-17A, by inducing a suite of Type 2 responses (Kim et al., 2002; Pan et al., 2001) and preventing IL-17A-dependent autoimmune disease (Kleinschek et al., 2007). Here, IL17A is linked to autoimmune disease.