The c-myc proto-oncogene is an early response gene which plays an important role in regulation of cell proliferation, differentiation and cell cycle control. Amplification and/or overexpression of this gene occurs in human breast carcinoma eels. The human c-myc promoter binding protein (MBP-1) is a negative regulatory factor for c-myc transcription, which binds to the upstream region of the c-myc P2 promoter. It was cloned and characterized from human cervical carcinoma cells. Normal human tissues and human breast carcinoma cells showed an elevated level of both MBP-1 and c-myc gene expression. In estrogen receptor (ER)-negative cells, the level of MBP-1 expression was higher than that of ER-positive cells. These results suggested a correlation between estrogen receptor status and the MBP-1 mRNA level.