in a surface groove of pRb. In the crystal structure, Leu-103 is positioned so that it makes van der Waals contacts with the hydrophobic side chains of Val-714 and Leu-769 in pRb. A number of hydrogen bonds also stabilize the TAg–pRb complex. For example, the side chain of Glu-107 forms hydrogen bonds by accepting hydrogens from the main chain amide groups of Phe-721 and Lys-722 in pRb. The mutation of Glu-107 to Lys-107 is expected to result in loss of these hydrogen bonds. Furthermore, the side chain of Lys-107 would likely have energetically unfavorable interactions with the amide of Phe-721 or Lys-722, destabilizing the complex. Strong experimental evidence confirms that positively charged amino acids (Lys or Arg) significantly weaken the binding interaction with pRB when positioned in the vicinity of the Leu – x – Cys – x – Glu sequence. This is likely due to the fact that the binding surface on pRb features six lysine residues, which will tend to repel positive residues within or flanking the Leu – x – Cys – x – Glu sequence. Of note, the highest-risk oncogenic human papillomavirus (HPV) strains (16, 18, 31, 45) encode E7 proteins featuring high-affinity pRb-binding domains which match the diagnostic pattern given above. References Viral oncoproteins Polyomavirus proteins