Leydig cells are the primary source of testosterone. As is true for all specialized cells, Leydig cells originally arise from primitive undifferentiated stem cells, stem Leydig cells (SLCs). The huge potential of SLCs is to be used therapeutically to restore testosterone levels in males with androgen deficiency due to diverse causes. The initial paradigm for cell therapy would call for harvesting the SLCs of an androgen deficient male, amplifying these cells in vitro, inducing differentiation in vitro, and then implanting the mature Leydig cells back into the same individual.