Acentric chromosome fragments may arise in a variety of ways. One way is that disrepair of DNA double-strand breaks can lead to symmetrical or asymmetrical chromatid and chromosome exchanges as well as chromatid and chromosome fragments. If DNA damage exceeds the repair capacity of the cell, unrepaired double-stranded DNA breaks may also result in acentric chromosome fragments. Another way eccentric chromosome fragments may arise is when defects in genes related to homologous recombinational repair (ex: ATM, BRCA1, BRCA2, and RAD51) result in a dysfunctional error-free homologous recombinational DNA repair pathway and causes the cell to resort to the error-prone non-homologous end-joining (NHEJ) repair pathway, increasing the likelihood of incorrect repair of DNA breaks, formation of dicentric chromosomes, and acentric chromosome fragments. If enzymes in the NHEJ repair pathway are defective as well, DNA breaks may not be repaired at all. Additionally, simultaneous excision repair of damaged or inappropriate bases incorporated in DNA that are in proximity and on opposite complementary DNA strands may lead to DNA double-stranded breaks and micronucleus formation, especially if the gap-filling step of the repair pathway is not completed.