Here, we present a case of a 50-year-old woman with stage IIIA invasive ductal carcinoma of the left breast who had genetic testing revealing pathogenic ATM mutations (c.5290del and c.4396C>G) and a PALB2 mutation (c.1619dup). While guidelines suggest that adjuvant radiation therapy is safe for patients with ATM mutations, this patient experienced severe radiation-induced toxicities, including brachial plexopathy. The gene discussed is ATM; the disease is invasive ductal breast carcinoma.