The physical consequences (phenotype) of the defective myostatin gene can be quite variable with some breeds (e.g. Belgian Blue and Piedmontese) exhibiting exaggerated muscling while others (e.g. South Devon) do not. Breeders need to be aware that there are significant problems associated with these phenotypic extremes, ranging from unacceptable levels of calving difficulty, through to inability to fatten, and reduced heat tolerance.