MSTN and muscular dystrophy: Caution should be exercised as to whether the inhibition of MSTN signaling benefits, as occurs in the ACVRB1 G allele, muscle function since, for instance, the downregulation of activin 1 receptor (Alk4) expression not only decreased skeletal muscle mass to an even lower level in mdx mice (model of muscular dystrophy) but also induced marked muscle atrophy in normal animals [51].