ADA and hyperinsulinemic hypoglycemia, familial, 4: Lymphopenia is a long-known ADR linked to the mechanism of action of Cladr that is preferentially activated inside the B and T lymphocytes interfering with the synthesis of DNA: Cladr is an analogue of deoxyadenosine and, for its resistance to adenosine deaminase (ADA)-mediated deamination, it consequently accumulates in the lymphocytes and determines the formation of the cytotoxic triphosphorylated Cladr which causes ADA deficiency and leads to the development of severe lymphopenia (Baker et al., 2019).