Within Dr. Harris’s experiments, tumor cells were fused with normal somatic cells to make hybrid cells. Each cell had chromosomes from both parents and upon growth, a majority of these hybrid cells did not have the capability of developing tumors within animals. The suppression of tumorigenicity in these hybrid cells prompted researchers to hypothesize that genes within the normal somatic cell had inhibitory actions to stop tumor growth. This initial hypothesis eventually lead to the discovery of the first classic tumor suppressor gene by Alfred Knudson, known as the Rb gene, which codes for the retinoblastoma tumor suppressor protein.