[Challenges to Hematopoietic Stem Cell Research Today].

More works documented recently indicated that human CD34(-) cells exist and are likely to be the precursors of the CD34(+) cells. Anyhow, the CD34(+) enriched populations have already been proved to show long-term reconstitution of hematopoiesis in animals and patients worldwidely. It still remains uncertain whether cells lack of CD34 and Lineage markers are the very best stem cells or maybe the residual embryonic stem cells keeping quiescent in the adult tissues are capable of transfer into hematopoietic stem cells when activated. Since just a negative selection technique is used to collect the CD34(-) cells everywhere for the time being, no final conclusion is convincing about the characterization of CD34(-) cells till a highly purified cell population of CD34(-)/Lin(-) is available. Clinical analysis shows that the most critical factor predicting the stem cell engraftment is the number of the cells infused. The number of nucleated cells in umbilical cord blood to be infused and required to obtain a successful engraftment is superior to 3.7 x 10(7)/kg. However, large dose of T-cell-depleted and purified CD34(+) cells as more than 5 x 10(6)/kg or even a 'megadose' of CD34(+) cells of 10(7)/kg is recommended for allotransplant of mobilized peripheral blood to achieve a high rate of successful engraftment. The delayed engraftment and the relapse of malignancy after cord blood transplantation are major problems. However, CBT is still the best choice of stem cell transplant for the baby patients with non-malignancies. At present, the HLA typing for class I antigens is still achieved with serology in most laboratories. As HLA typing is increasingly defined to higher degree of resolution by DNA probes, it is recommended to check with genotyping when there is a 'match' by the serological phenotyping especially for the unrelated donor/recipient couples. Improvements in DNA-based methods for the detection of numerous HLA alleles have provided the opportunity to investigate the relationship between HLA disparity and transplant complications. About 80% (or even more) of patients in China who might benefit from stem cell transplantation still fail to find suitable donors. It is worthy to adopt the unrelated donors matched or mismatched for those high-risk acute leukemia patients who do not have related matched donors but urgently need transplant. The advanced experience of unrelated mismatched transplant will no doubt be certain to carry weight and be disseminated in China. A great leap forward of the clinical practice and biological study on hematopoietic stem/progenitor cells is expected in the new century to accept the challenge from the world outside China.