The present invention relates to means and methods of diagnosing and treating the phenotypic spectrum as well as the overlapping clinical characteristics with several forms of inherited abnormal expression and/or function of the CYP2D6 gene. In particular, the present invention relates to polynucleotides of molecular variants of the CYP2D6 gene, which for example, are associated with abnormal drug response and disorders caused by altered activity of the CYP2D6 enzyme, and vectors comprising these polynucleotides. Furthermore, the present invention relates to host cells comprising such polynucleotides or vectors. The invention also relates to a transgenic non-human animal. Furthermore, kits comprising oligonucleotides hybridizing to the CYP2D6 gene useful for the diagnosis of an altered activity of the CYP2D6 enzyme are provided. In addition, the invention relates to methods for diagnosing a polynucleotide associated with an intermediate metabolizer phenotype of CYP2D6 drugs.