While significant literature exists regarding the link between ATM mutations, breast cancer incidence, and radiation sensitivity, this paper reports on a severe case of radiation-induced brachial plexopathy and other likely radiation-related toxicities in the setting of ATM variant c.5290del (p.Leu1764TyrFs*12) and ATM variant c.4396C>G (p.Arg1466Gly) in the setting of PALB2 variant c.1619dup (p.Asn540Lysfs*38). These ATM mutations have not previously been described as imparting severe radiation-associated toxicities. This evidence concerns the gene ATM and breast cancer.