The 65-kDa oncofetal protein (p65), a novel tumor marker (1 –7 ), is highly conserved m different species (2 ,4 ). We have identified the p65 gene as a novel member of the family of genes that encode receptors for steroid hormones, vitamin D, retmoic acid and thyroid hormone (7 ). The p65 protein is highly homologous to estrogen receptor (ER) m its DNA binding domain but other regions of the sequence do not show similarities, indicating that p65 is a new transcription factor or receptor with an as yet unknown ligand. Using enzymelmked immunosorbent assay (ELISA) and immunostaining, p65 was shown (5 ,6 ), to be a promismg marker for diagnosis and prognosis of cancer. With a view toward early detection of cancer, we have studied strain-dependent differences in the expression of p65 in different organs of mice highly susceptible (C3H/HeJ) and relatively resistant (C57BL/6N) to carcinogenesis. In this chapter, we describe a new, general procedure for detection of p65 mRNA by reverse transcription polymerase chain reaction (RT-PCR). This method is sensitive enough to detect a small number of the p65-specific mRNA molecules in mammary glands and livers of young, 7–8 wk-old female mice of the C3H/HeJ strain are known to spontaneously develop mammary tumors and be sensitive to liver tumor induction. No p65-specific mRNA was detected in a control group of C57BL/6N mice known to be resistant to chemical carcmogenesis.