The oculo-auriculo-vertebral spectrum (OAVS) (OMIM % 164210) is a common developmental disorder characterized by hemifacial microsomia, epibulbar tumours, ear malformation and vertebral anomalies. Although rare familial cases suggest that OAVS has a genetic basis, no genetic defect has been identified so far. In a patient with OAVS and a chromosomal translocation t(4;8) we have found that the chromosome 4 breakpoint is 76.4 kb distal to the BAPX1 gene, which plays an essential role in craniofacial development. We did not detect any BAPX1 mutation in 105 patients, but observed a strong allelic expression imbalance (sAEI) in fibroblasts from five of 12 patients, but not in nine normal controls (Fisher's exact test, P=0.038). sAEI was de novo in one patient and inherited in two other patients. Prolonged cell culture or treatment with the histone deacetylase inhibitor Trichostatin A led to reactivation of the downregulated allele. We propose that epigenetic dysregulation of BAPX1 plays an important role in OAVS.