In the present study, Myostatin gene expression in the LL and ST muscles at 19 months of age was higher in the GHF group compared with the CF group; this finding indicates that the skeletal muscle growth in the GHF group is suppressed around this time by Myostatin. In contrast, despite being measured before conversion from grass hay diet to concentrate diet, Myostatin expression in the LL muscle at 22 months of age was lower in the GHF group compared with the CF group; this finding suggests that skeletal muscle in the GHF group was activated by Myostatin at this time. This evidence concerns the gene MSTN and cystic fibrosis.