When DNA is damaged, cells have immediate responses such as DNA repair and halting the cell cycle to protect and aid in recovery. However, if the damage is severe and cannot be repaired adequately, the cell may trigger apoptosis or senescence at a later stage. These delayed responses are mechanisms to eliminate the damaged cell from the organism. DNA damage, although different from mutation, can lead to mutations if left unrepaired. While repair mechanisms exist, they are not perfect and some damage may remain unrepaired. Accumulation of unrepaired DNA damage in non-dividing cells (e.g. muscle cells or neurons) can contribute to the aging process.