A transcriptional enhancer with specificity for erythroid cells is located in the long terminal repeat of the Friend murine leukemia virus.

We investigated the ability of the U3 region of the long terminal repeats (LTR) of the Friend murine leukemia virus (Fr-MuLV) and Moloney murine leukemia virus (Mo-MuLV) to promote transcription in a variety of human cell lines. Our analysis reveals the presence of a transcriptional enhancer with specificity for erythroid cells in the U3 region of the Fr-MuLV. This constitutes the first example of an enhancer with such a property. Analysis of the Mo-MuLV enhancer suggests that it is active at least in erythroid and lymphoid cells and has thus a less restricted specificity than the Fr-MuLV enhancer. The different tissue specificities of the two enhancers correlate with the different tissue selectivities and pathogenic properties of the two viruses.