Mutations in the myostatin (GDF8) gene are responsible for the double-muscling trait in Belgian Blue and Piedmontese cattle. This trait is characterized by increased muscle mass and fiber number, with minimal enlargement of individual fibers. The myostatin gene, a member of the TGF-β superfamily, is located on bovine chromosome 2 and is closely linked to the double-muscling locus (mh). In Belgian Blue cattle, a 11-bp deletion in the coding region leads to a frameshift mutation, resulting in a truncated protein that lacks a conserved region critical for function. In Piedmontese cattle, a G-A transition in exon 3 changes a conserved cysteine residue to tyrosine, potentially disrupting protein function. These mutations are likely responsible for the double-muscling phenotype, as they affect the activity of myostatin, a negative regulator of muscle growth. Knockout mice lacking myostatin also exhibit increased muscle mass, supporting the role of myostatin in regulating muscle growth. Despite similar phenotypes, differences in muscle growth mechanisms between cattle and mice suggest additional genetic factors may be involved. The study confirms that myostatin is the gene responsible for the double-muscling trait in these cattle breeds.Mutations in the myostatin (GDF8) gene are responsible for the double-muscling trait in Belgian Blue and Piedmontese cattle. This trait is characterized by increased muscle mass and fiber number, with minimal enlargement of individual fibers. The myostatin gene, a member of the TGF-β superfamily, is located on bovine chromosome 2 and is closely linked to the double-muscling locus (mh). In Belgian Blue cattle, a 11-bp deletion in the coding region leads to a frameshift mutation, resulting in a truncated protein that lacks a conserved region critical for function. In Piedmontese cattle, a G-A transition in exon 3 changes a conserved cysteine residue to tyrosine, potentially disrupting protein function. These mutations are likely responsible for the double-muscling phenotype, as they affect the activity of myostatin, a negative regulator of muscle growth. Knockout mice lacking myostatin also exhibit increased muscle mass, supporting the role of myostatin in regulating muscle growth. Despite similar phenotypes, differences in muscle growth mechanisms between cattle and mice suggest additional genetic factors may be involved. The study confirms that myostatin is the gene responsible for the double-muscling trait in these cattle breeds.