Despite the wealth of research on stem-cell biology and therapy, there have been few advances in the ex vivo expansion of adult stem cells. Fetal bovine serum, which can elicit an immune reaction in hosts receiving cellular products, remains essential to the culture methods. Recent work by Prockop et al. suggests that Dickkopf-1 (Dkk-1) could stimulate the proliferation of mesenchymal stem cells (MSCs) while maintaining an undifferentiated phenotype. The use of Dkk-1 in cell-processing laboratories might advance cell-expansion technology and greatly increase the clinical uses of MSCs.