BRCA1 deficiency is commonly associated with the decreased expression of key factors in alternative NHEJ and base excision repair (BER), specifically apurinic/apyrimidinic endonuclease 1 (APE1), DNA polymerase b (POLB), and X-ray repair cross-complementing 1 (XRCC1) [116]. Here, APEX1 is linked to hyperinsulinemic hypoglycemia, familial, 4.