| We show that small interfering RNA mediated knockdown of DDB 1 , CSN 1 ( a subunit of the signalosome ) , and Cul4A in mammalian cells causes an accumulation of p27Kip1 . ^^^ The DDB 1 induced proteolysis of p27Kip1 requires signalosome and Cul4A , because DDB 1 failed to increase the decay rate of p27Kip1 in cells deficient in CSN 1 or Cul4A . ^^^ |