Mutations in EFHC1 gene have been previously reported in patients with epilepsies, including those with juvenile myoclonic epilepsy. Myoclonin1, also known as mRib72-1, is encoded by the mouse Efhc1 gene. Myoclonin1 is dominantly expressed in embryonic choroid plexus, post-natal ependymal cilia, tracheal cilia and sperm flagella. In this study, we generated viable Efhc1-deficient mice. Most of the mice were normal in outward appearance, and both sexes were found to be fertile. However, the ventricles of the brains were significantly enlarged in the null mutants, but not in the heterozygotes. Although the ciliary structure was found intact, the ciliary beating frequency was significantly reduced in null mutants. In adult stages, both the heterozygous and null mutants developed frequent spontaneous myoclonus. Furthermore, the threshold of seizures induced by pentylenetetrazol was significantly reduced in both heterozygous and null mutants. These observations seem to further suggest that decrease or loss of function of myoclonin1 may be the molecular basis for epilepsies caused by EFHC1 mutations.