Inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia (IBMPFD) is a dominant progressive disorder that maps to chromosome 9p21.1-p12. We investigated 13 families with IBMPFD linked to chromosome 9 using a candidate-gene approach. We found six missense mutations in the gene encoding valosincontaining protein (VCP, a member of the AAA-ATPase superfamily) exclusively in all 61 affected individuals. Haplotype analysis indicated that descent from two founders in two separate North American kindreds accounted for IBMPFD in ~50% of affected families. VCP is associated with a variety of cellular activities, including cell cycle control, membrane fusion and the ubiquitin-proteasome degradation pathway. Identification of VCP as causing IBMPFD has important implications for other inclusion-body diseases, including myopathies, dementias and Paget disease of bone (PDB), as it may define a new common pathological ubiquitin-based pathway.Hereditary inclusion body myopathy (IBM) associated with PDB and frontotemporal dementia (FTD), or IBMPFD, is a rare, complex and ultimately lethal autosomal dominant disorder (OMIM 605382; ref. 1). IBMPFD features adult-onset proximal and distal muscle weakness (clinically resembling limb girdle muscular dystrophy), early-onset PDB in most cases and 'premature' FTD 2 . Although the disorder was mapped to chromosome 9p21-p12, the genetic basis was not known.Within 13 families (12 from the United States and 1 from Canada, Fig. 1 and Supplementary Fig. 1 online), 82% of individuals had myopathy, 49% had PDB and 30% had early-onset FTD. The mean age of presentation was 42 years for both IBM and PDB, whereas FTD typically presented at age 53 years. In IBMPFD myopathic muscle and PDB osteoclasts, inclusions appear similar, suggestive of disruptions in the same pathological pathway.Haplotype analysis of the families with IBMPFD identified two ancestral, disease-associated haplotypes, distinguishing families 1, 3, 7 and 16 (group A) from families 2 and 5 (group B), both groups of Northern European ancestry ( Table 1