Paratarg-7 (P-7) is a frequent paraprotein target in monoclonal gammopathy of undetermined significance (MGUS), multiple myeloma (MM), and Waldenström macroglobulinemia. Patients with P-7-specific paraproteins carry a hyperphosphorylated paratarg-7 (pP-7). Because pP-7 carrier state is dominantly inherited, we determined the paraprotein targets in 4 families with familial MGUS/MM. No antigenic target was identified for the paraproteins from 2 members of one family. Paraproteins from affected members of 2 other families targeted P-7, and paraproteins from 4 affected members of a fourth family targeted P-8, which is encoded by the ATG13 gene. P-8 was hyperphosphorylated in the affected family members (pP-8) and pP-8 carrier state is inherited in a dominant fashion. Six additional autoantigenic nonfamilial paraprotein targets were also hyperphosphorylated in the respective patients compared with normal controls. We conclude that paraproteins of affected members with familial MGUS/MM share family-typical hyperphosphorylated antigens and hyperphosphorylation of paraprotein targets might be a general mechanism underlying the pathogenesis of MGUS/MM.