Ex vivo expansion of hematopoietic stem cells (HSCs) is considered the holy grail in stem cell biology and therapy, as it has long been difficult to make this procedure possible. Yamazaki's research team has established new, polyvinyl alcohol-based culture conditions and shown a significant expansion of mouse HSCs from a small number of cells after a month of culture. Surprisingly, expanded HSCs were able to reconstitute unconditioned normal mice. There is generally a technical concern in limiting dilution assay to estimate a fold-expansion of HSCs. But, this work paves the way toward expansion of human HSCs useful for transplantation medicine.