Homozygous mice harbouring the recurrent human variant NM_024854.5:c.464A > G;p.(N155S), termed Pyroxd1N155S, phenocopied a severe PYROXD1 disorder, presenting from ~ 10 weeks of age with a progressive myopathy, myofibrillar disorganisation, decreased contractile strength, muscle hypotrophy and osteopenia. The gene discussed is PYROXD1; the disease is Osteopenia.