Abstract: A vector for gene expression comprises a promoter polynucleotide sequence, a polynucleotide sequence encoding a first detectable protein in a first reading frame, a polynucleotide sequence adapted for insertion of a test sequence, a nonsense-mediated decay polynucleotide sequence, a start codon that is in a second reading frame, a polynucleotide sequence encoding a second detectable protein in the first reading frame. A polynucleotide sequence encoding a third detectable protein is optionally present. The second reading frame may be one or two reading frames removed from the first reading frame. The polynucleotide sequence encoding a third detectable protein may be flanked on the 5 and 3? ends by recombination sequences. A method of detectably inducing expression of a gene utilizes the vector.