In Vitro: The p53 protein, referred to as the “guardian of the human genome” , is a tetrameric transcription factor that prevents mutation to the genome by regulating the expression of a subgroup of target genes. Although biologically active as a homotetramer, each p53 monomer is comprised of 393 amino acids, and is divided into five key regulatory domains: the transactivation domain (TAD) , proline-rich region (PR) , the DNA binding domain (DBD) , the oligomerization domain (OD) , and the C-terminus.