Objective    To study the effects of Genistein (Gen) on the proliferation of human breast cancer cell lines in vitro and the relationship with telomerase.     Methods    Human breast cancer cell lines MCF-7 (ER

+

) and MDA-MB-231(ER

-

) were cultured in vitro. MTT assay was used to determine the effects of Gen and Estradiol (E

2

) on the proliferation of the two cell lines. RT-PCR was conducted to detect the expression of TERT mRNA and  immunocytochemistry assay was used to observe the nuclear translo

cation of NF-κB.     Results    E

2

promoted the proliferation of MCF-7 cells (P0.01)

the expression of TERT mRNA (P0.01) and the nuclear translocation of NF-κB

but had no effect on MDA-MB-231 cells. The effect of Gen alone on  MCF-7 in low dose was similar to E

2

and the effect of Gen in combination with E

2

was similar to the control. For both cell lines

in high dose

Gen inhibited the proliferation(P0.01)

the expression of TERT mRNA (P0.01) and the nuclear translocation of  NF-κB p65 when used alone or in combination with E

2

.     Conclusion    The varied effects of Gen on MCF-7 and MDA-MB-231 cells are related to the status of ER expression and the presence or absence of estrogen

and the underlying mechanisms of these effects may be through regulating the TERT expression and nuclear translocation of NF-κB p65.