OBJECTIVE: To identify the genetic cause of autosomal dominant Familial Focal Epilepsy with Variable Foci (FFEVF), to investigate the prevalence of mutations in the FFEVF causative gene in familial cases of non-lesional focal epilepsy, to study the expression in the brain and the subcellular localization of the encoded protein. BACKGROUND: FFEVF is characterized by seizures arising from different cortical regions in different affected family members. Brain imaging is normal. Seizure onset varies from infancy to adult life. Affected individuals occasionally have neuropsychiatric co-morbidities. Linkage studies mapped FFEVF to chromosome 22q12, but the causative gene had so far eluded identification. DESIGN/METHODS: We applied exome sequencing to two FFEVF families previously linked to chromosome 22q12, identifying DEPDC5 as the most likely candidate gene. We sequenced DEPDC5 in six additional 22q12-linked families and scanned DEPDC5 for sequence variation in 82 unrelated probands from families with at least two individuals with non-lesional focal epilepsy. We used qRT-PCR, immunofluorescence and western blot analysis to study DEPDC5 expression and subcellular localization. RESULTS: Heterozygous mutations in DEPDC5 were identified in 7/8 FFEVF families linked to chromosome 22q12 and in 10/82 (12.2%) probands from the small families with focal epilepsy. Each DEPDC5 mutation segregated with the FFEVF phenotype in the respective family and was absent in both dbSNP135 and an in-house exome sequencing database of 710 chromosomes. Most mutations caused premature termination codons suggesting haploinsufficiency as pathogenic mechanism. DEPDC5 encodes a1604 amino acid protein of unknown function, probably implicated in modulation of intracellular signaling.Mouse Depdc5 transcripts were detected at low levels in all brain regions and throughout brain development. Immunofluorescence analyses in mouse and human brain showed specific expression in neurons and perinuclear localization. CONCLUSIONS: Our findings establish DEPDC5 mutations as the most common known cause of familial focal epilepsy and identify a novel pathogenic pathway for epilepsy.