challenge: What is the primary function of DDB1 in the cell?
  
  A. To directly repair damaged DNA
  B. To catalyze the formation of covalent bonds between amino acids
  C. To serve as a core component of the CUL4A- and CUL4B-based E3 ubiquitin ligase complexes
  D. To regulate gene expression by binding to specific DNA sequences
  Answer: 