The study investigates the genetic basis of double muscling in Belgian Blue and Piedmontese cattle, which is characterized by an increased number of muscle fibers. The *mh* locus, responsible for this condition, maps to bovine chromosome 2, overlapping with the *myostatin* gene, a member of the TGF-β superfamily. The researchers cloned the bovine myostatin cDNA and analyzed its expression and sequence in normal and double-muscled cattle. They found that the levels and timing of *myostatin* expression were similar in both groups, suggesting that the *mh* allele does not affect mRNA expression. However, mutations were identified in the coding region of the *myostatin* gene in both breeds. Belgian Blue cattle were found to be homozygous for an 81-bp deletion that results in a frameshift mutation, while Piedmontese cattle had a G-A transition that changes a conserved cysteine to tyrosine. These mutations likely impair the function of Myostatin, a negative regulator of muscle growth, in these breeds. The findings suggest that *myostatin* is the *mh* locus responsible for double muscling in cattle.The study investigates the genetic basis of double muscling in Belgian Blue and Piedmontese cattle, which is characterized by an increased number of muscle fibers. The *mh* locus, responsible for this condition, maps to bovine chromosome 2, overlapping with the *myostatin* gene, a member of the TGF-β superfamily. The researchers cloned the bovine myostatin cDNA and analyzed its expression and sequence in normal and double-muscled cattle. They found that the levels and timing of *myostatin* expression were similar in both groups, suggesting that the *mh* allele does not affect mRNA expression. However, mutations were identified in the coding region of the *myostatin* gene in both breeds. Belgian Blue cattle were found to be homozygous for an 81-bp deletion that results in a frameshift mutation, while Piedmontese cattle had a G-A transition that changes a conserved cysteine to tyrosine. These mutations likely impair the function of Myostatin, a negative regulator of muscle growth, in these breeds. The findings suggest that *myostatin* is the *mh* locus responsible for double muscling in cattle.