The c-myc oncogene was not rearranged in any of 5 human breast carcinoma cell lines [BT-20, SKBR-3, MCF-7, CAMA-1, and SKBR-5] examined. A 10-fold amplification and an elevated expression of the oncogene were detected in 1 cell line, and the 4 other lines expressed low or undetectable levels of c-myc transcripts. Thus, c-myc amplification and elevated expression may be found only in a minority of human breast carcinomas.