Mutations in WNT1 are associated withosteogenesis imperfecta and with early onset osteoporosis (Fahiminiya et al.2013; Keupp et al. 2013; Joeng et al. 2014), which isphenocopied in mice carrying the human WNT1 mutation orwith a cell-specific deletion of Wnt1 in the osteoblastlineage. The gene discussed is WNT1; the disease is osteoporosis.