Cullin-RING ligases (CRLs) complexes participate in the regulation of diverse cellular processes,including cell cycle progression,transcription,signal transduction and development . Serving as the scaffold protein,cullins are crucial for the assembly of ligase complexes,which recognize and target various substrates for proteosomal degradation. Two cullin 4 (CUL4) proteins,CUL4A (87 kDa) and CUL4B(104 kDa),are two members in cullin family with 83% of identity. Mutations in human CUL4B are one of the major causes of X-linked mental retardation. Cul4b knockout mice demonstrated that CUL4B is indispensable for embryonic development in the mouse .