The gene for bovine leukemia virus (BLV) reverse transcriptase was cloned in prokaryotic expression vector pUC8-2. After fusion of Escherichia coli lacZ gene to different parts of reverse transcriptase we detected expression of new proteins with molecular weights corresponding to the size of the hybrid genes. A coding region most probably responsible for about a hundred-fold decrease in expression of long fusion proteins has been identified. A few possible causes of this phenomenon were tested.