MG is a common event of hematologic malignancies. There are many papers regarding kidney transplantation patients with MGUS in adults, while data in pediatrics are scarce. The etiology and clinical significance of MGUS are unclear both in adults and children. Immunosuppressive drugs, graft antigenicity, and viral infection could play a possible role. The viruses most frequently implicated seem to be EBV or CMV in particular, but their role has to be defined better. However, many investigators have emphasized an impaired balance between an adequate immune response and reactivation of viral infection.