GADD45A and muscle atrophy: These facts suggest that HDAC4/Gadd45α axis plays an important role in hindlimb unloading-induced muscle atrophy in aged skeletal muscle; thus, HDAC4/Gadd45α axis may be a key pathway for developing potential therapeutic countermeasures in aged muscles before muscle disuse and in identifying target molecules to protect against disuse skeletal muscle atrophy.