example in u s pat no 5 252 479 and in wipo publications wo 92 07573 wo 90 06997 wo 89 05345 wo 92 05266 and wo 92 14829 incorporated herein by reference which provide a description of methods for efficiently introducing nucleic acids into human cells using such retroviral vectors other retroviral vectors include for example the mmtv vectors e g shackleford et al 1988 proc nat 39 l acad sci usa 85 9655 9659 and the like the following examples illustrate the biological activity of iebp as well as methods for preparing cells and cell lines that produce and or overexpress iebp in the following examples where indicated experimental means were compared statistically using an unpaired student 39 s t test all cell lines were obtained from american type culture collection atcc rockville md the estrogen resistant nwp cell line b95 8 derived from the hormone resistant common marmoset callithrix jacchus was maintained in rpmi 1640 medium the estrogen responsive owp breast cell line 6299 derived from a rhesus monkey macaca mulatta was maintained in dmem irvine scientific irvine calif all cultures were routinely supplemented with 10 fetal calf serum fcsi gemini bioproducts calabasas calif 100 units ml penicillin 100 u03bcg ml streptomycin 2 mm l glutamine both from gibco brl grand island n y and in atmosphere of 95 air 5 co 2 in some experiments confluent cultures were preincubated for up to 48 hours in medium containing e 2 10 nm prior to harvest and preparation of extracts postnuclear.