At first, this was a reaonable belief: the protein is with viral transforming factor, and cloned p53 DNA from normal cells was able to transform 3T3 cells when over-expressed. It showed exactly what oncogenes are supposed to be. But the cloning procedure, as occasionally happens, had mutated the cloned sequence, and, by great misfortune, the mutation was a dominant one, in which the mutant protein overcomes the effect of normal proteins.
Later, normal unmutated p53 was found and with no transforming activity. It prevents the growth of tumor if over-expressed; and several tumors were found to show loss of heterozygosity at 17p13.1. Therefore, p53 was re-interpreted as a tumor supressor gene.