Nuclear factor kB, nuclear factor interleukin-6 (NFIL6 or C/EBPbeta) and nuclear factor interleukin-6beta (NFIL6beta or C/EBPdelta) are not sufficient to activate the endogenous interleukin-6 gene in the human breast carcinoma cell line MCF-7. Comparative analysis with MDA-MB-231 cells, an interleukin-6 expressing human breast carcinoma cell line.