HPV protein bound to tumour suppressor, molecular model. The human papilloma virus (HPV) produces a protein, E7, that binds to retinoblastoma tumour suppressors. These are proteins produced by the cell to encourage normal cell cycles, thus preventing the formation of tumours. When E7 binds, the tumour suppressor's action is blocked, and the development of malignant tumours becomes more likely. This model shows the two proteins forming the tumour suppressor pocket and the single E7 protein. The solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices), and the faded tubular structure behind represents the individual atoms and bonds.