Abstract: There is provided a vector comprising a first promoter sequence which constitutively or conditionally regulates transcription, a first transcription structure including a DNA-binding region, a nucleus localization signal sequence, an AhR-ligand binding control region and a transcriptional activation region, which is arranged downstream of the first promoter sequence, one or more second promoter sequence which is specifically coupled with the DNA-binding region, thereby to transcribe a transcription unit under control of the transcriptional activation region, and a second transcription structure including a reporter gene, which is arranged downstream of the second promoter sequence. Also, there are provided a transformant with the vector, and a method for monitoring and/or reducing the AhR-ligand.