Ex vivo maintainance of human stem cells is crucial for many clinical applications. Current culture conditions provide some level support but cytokines induce most quiescent stem cells to proliferate and differentiate. Better control of primitive cells is needed to extend the time and range of manipulation of such cells. A recently identified plant lectin Flt3 receptor-interacting lectin (FRIL) present may a special ability to preserve primitive CB progenitors for extended periods in culture without exogenous cytokines. But the mechanisms of FRIL preserving quiescent primitive cells are still unknown. Recently a novel protein HTm4 and its alternatively spliced variant HTm4S, which serve as hematopoietic cell cycle regulators, have been identified. In this report we studied the effect of FRIL on the in vitro maintenance of quiescent human cord blood stem cells and the expression of the novel hematopoietic cell cycle regulator HTm4 and HTm4S in progenitor cells cultured in FRIL. We analyzed the proliferation and the HPP-CFC proportion of CD34(+) cells treated with FRIL. The human HTm4 and HTm4S mRNA expression was detected by semi-quantitative RT-PCR, and the cell cycle status of CB CD34(+) cells was analyzed by FACS. The results showed that incubation of CD34(+) cells in FRIL resulted in a low proliferation of progenitor cells and fewer cycling cells, but FRIL selectively maintained a higher number of primitive cells with proliferative potential in suspension culture. CB CD34(+) cells cultured in FRIL showed significant diversity in the expression of HTm4 and HTm4S during 0~14 d. On d 0, HTm4 was detected at high level, downregulated on d 1, but upregulated during d 3 to d 14, and reaching the highest level on d 7. But the expression levels of HTm4S changed little in the cells cultured in FRIL except the obviously increased expression on d 7. Exogenous expression showed that HTm4 was localized around the karyon while HTm4S scatted in the cytoplasm, respectively, which may be responsible for their difference in function. Thus, FRIL can preserve quiescent primitive CD34(+), and FRIL's ability to preserve quiescent primitive cells in a reversible manner may significantly expand the time and range of ex vivo manipulations of human stem cells for clinical applications. In other words, HTm4 and HTm4S may play a crucial role in the cell cycle modulation of CD34(+) progenitor cells maintained with FRIL in vitro.