Specificity of gene expression in tumor cells

We have isolated gene sequences which are expressed abundantly in tumor cells but must less so in normal tissues from a cDNA clone library that was constructed using poly (A)+ RNAs from azo-dye-induced rat ascites hepatomas. These clones were categorized into 2 groups, with respect to their size distribution of mRNAs from which the clones were derived. The first group was complementary to a single distinct species of poly (A)+ RNA, and was considered to be derived from protein-coding genes. The second group showed no distinct bands but a smear on the RNA blot, and was shown to be transcribed from middle repetitive sequences of DNA. On the other hand, a certain species of highly repetitive sequences is also expressed in many lines of ascites hepatomas. Semiquantitative RNA dot blot assays have revealed the sequences of these clones to have a very low level of expression, if at all, in normal and regenerating livers, while being generally high in many species of tumors. The transcription of these gene sequences is considered to be closely related to the establishment of tumor phenotypes.