Nucleic acid constructs containing hybrid promoters for use in gene therapy and genetic manipulation. The invention relates to a nucleic acid construct for the precise, regulated expression of genes in host cells, which construct exhibits at least one mutation which inhibits the proper expression of the expressed gene and exhibits at least one additional second mutation which relieves the inhibition due to the first mutation, to an isolated cell which harbors the nucleic acid construct, and to the use of the nucleic acid construct for preparing pharmaceuticals and for treating diseases with excessive cell proliferation.