Mammalian progenitor or stem cells are expanded in vitro by increasing the levels of &bgr;-catenin in the cell. The expanded cells substantially maintain their original phenotype including the ability to give rise to multiple types of differentiated cells. The intracellular levels of &bgr;-catenin may be manipulated by providing exogenous &bgr;-catenin protein to the cell, or by introduction into the cell of a genetic construct encoding &bgr;-catenin. The &bgr;-catenin may be a wild-type or stabilized mutant form of the protein. Preferably the long term cell culture medium substantially lacks stromal cells and cytokines.