In a study comparing the release of inflammatory cytokines known to initiate and strengthen the immune response, interferon-gamma (IFN-γ) and interleukin (IL)-17A, in specific-pathogen-free (SPF) and germ-free (GF) mice, it was found that T cells of GF mice produced significantly less IFN-γ and IL-17A compared with SPF mice, resulting in impaired skin immunity essential for controlling infection by other pathogens in GF mice [55]. The gene discussed is IL17A; the disease is infection.