Abstract: Described are general 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 Multi Drug Resistance-1 (MDR-1) gene. In particular, polynucleotides of molecular variant MDR-1 genes which, for example, are associated with unsufficient and/or altered uptake of drugs by a target cell, and vectors comprising such polynucleotides are provided. Furthermore, host cells comprising such polynucleotides or vectors and their use for the production of variant MDR-1 proteins are described. In addition, variant MDR-1 proteins and antibodies specifically recognizing such proteins as well as concerns transgenic non-human animals comprising the above-described polynucleotide or vectors are provided.