LIG4 and neoplasm: In defects of 1st step (RAG1, RAG2), abnormalities are limited to T and B lymphocytes; by contrast, defects of 2nd step (NHEJ) affect all cells and cause problems similar to other syndromes with DNA repair defects: cellular radiosensitivity, extreme toxicity by pre-HSCT conditioning (especially by alkylating agents and, obviously, by irradiation), predisposition to neoplasia, and (but only in some forms: defect of DNA ligase IV, defect of Cernunnos/XLF) dysmorphic facies, microcephaly, psychomotor delay [62].