Most of the therapeutic mechanism of regenerative medicine action was defined and wide-ranging toxicology studies were done for stem cells therapy. Also, with the improvement of proper standardization and characterization of transplanted cell preparations, stem cell therapies have demonstrated therapeutic efficacy and benefit in preclinical models through paracrine mechanisms or differentiation. Since target disease will probably influence which of these mechanisms is more important, the stem cells which specific to different diseases are necessary to advance the field.