We have previously found linkage to chromosome 1p34 in five large families with autosomal dominant non-syndromic hearing impairment (DFNA2). In all five families, the connexin31 gene ( GJB3 ), located at 1p34 and responsible for non-syndromic autosomal dominant hearing loss in two small Chinese families, has been excluded as the responsible gene. Recently, a fourth member of the KCNQ branch of the K+channel family, KCNQ4, has been cloned. KCNQ4 was mapped to chromosome 1p34 and a single mutation was found in three patients from a small French family with non-syndromic autosomal dominant hearing loss. In this study, we have analysed the KCNQ4 gene for mutations in our five DFNA2 families. Missense mutations altering conserved amino acids were found in three families and an inactivating deletion was present in a fourth family. No KCNQ4 mutation could be found in a single DFNA2 family of Indonesian origin. These results indicate that at least two and possibly three genes responsible for hearing impairment are located close together on chromosome 1p34 and suggest that KCNQ4 mutations may be a relatively frequent cause of autosomal dominant hearing loss.