A common key factor in obesity is the low abundance of A. muciniphila (Schneeberger et al., 2015; Palleja et al., 2016; Medina et al., 2017; Seck et al., 2018), a mucin-degrading bacterium that resides in the human gut mucus layer and whose abundance in healthy subjects represents 3–5% of the residential microbial community (Derrien et al., 2004). The gene discussed is MUC5AC; the disease is obesity due to melanocortin 4 receptor deficiency.