The present invention relates to modified cells carrying a heterologous gene sequence encoding a protein, such as an Inhibitor of differentiation (Id) gene sequence that binds a basic helix-loop-helix (bHLH) protein to inhibit cell growth, differentiation and/or tumorigenesis of the modified cells. The modified cells are differentiated, proliferate and do not become tumorigenic when grafted into a recipient subject. Additionally, the modified cells produce a factor or factors that enhance the viability of co-grafted organs, tissues or cells. Thus, the modified cells are useful for testing agents for effects on the cells, for co-grafting with transplant organs, tissues or cells. The modified cells are also useful for enhancing the viability of thawing cells that have been cryo-preserved. In one embodiment, the modified cells are modified Sertoli cells.
