The present invention relates to a biological entity, notably a rat, carrying a regulator construct comprising a specific repressor gene and a responder construct comprising at least one segment corresponding to a short hairpin RNA (shRNA) or corresponding to complementary short interfering RNA (siRNA) strands or corresponding to miRNA, said at least one segment being under control of a promoter which contains an operator sequence corresponding to the repressor. The invention further relates to a method for preparing said biological entity and its use.