Undifferentiated primordial stem cells are manipulated to permit their long term growth in defined media lacking serum and feeder layer cells by shifting the apoptotic balance of the cells, through increasing the activity of Bcl-2 family anti-apoptotic proteins or decreasing the activity of Bcl-2 family pro-apoptotic proteins. In some embodiments of the invention, the Bcl family protein is Bcl-2. The ES cells sustain the characteristics of undifferentiated, pluripotent stem cells during long-term serum- and feeder layer cell-free conditions, including the ability to be expanded in vitro, but maintain their potential to differentiate into mature cell types.