This is exemplified by the ability of only IL-4 to differentiate naïve CD4+ T cells into Th2 cells (Seder & Paul, 1994) or the non-redundant role of IL-13 in parasite expulsion, allergic inflammation and asthma (Bancroft et al, 1998; Grunig et al, 1998; Liang et al, 2011). The gene discussed is IL4; the disease is asthma.