CLOCK and obesity due to melanocortin 4 receptor deficiency: In general, GF mice are resistant to HFD-induced obesity, produce lower levels of inflammatory cytokines including TNFα, IFNγ, IL-1B, and IL-17 [41], and have blunted transcriptional oscillations of the core clock gene Bmal1 [38] relative to conventionally-raised specific pathogen-free (SPF) mice.