Mice lacking Muc2 (Van der Sluis et al, 2006), Agr2 (Park et al, 2009; Zhao et al, 2010) or having mutations in Muc2 that interfere with initial mucin folding and assembly (Heazlewood et al, 2008) develop spontaneous colitis or display rectal prolapse. The gene discussed is MUC5AC; the disease is colitis.