The important paper by Hahn and Weinberg extended the early studies from their lab and others in which mouse 3T3 cells were found to be transformed by single oncogenes (like mutant ras). What was the key difference in their experimental system that led them to conclude that at least 3 changes (expression of telomerase, presence of an oncogene like mutant ras, and decreased function of p53/Rb via SV40 Large T antigen) were required to transform non-immortalized cells