All living organisms and ecosystems are permanently exposed to ionizing radiation.Of all the types of ionizing radiation,heavy ions such as 56Fe have the potential to cause the most severe biological effects.We therefore examined the effects and potential mechanisms of iron ion irradiation on the induction of apoptosis in zebrafish embryos.Numerous experiments have shown that high-LET heavy ions may cause more DNA breaks than low-LET radiation.When DNA damage is irreparable,cell death occurs.DNA fragmentation is a characteristic feature of apoptosis,which is considered to be one consequence of radiation,via the induction of programmed cell death.