The present invention relates to isolated polypeptides which comprise a functional kinase domain comprising the amino acid sequence of the native human Abl kinase domain or an essentially similar sequence thereof in which at least one amino acid selected from Met244, Leu248, Gly250, Glu252, Tyr253, Val256, Glu258, Phe311, Ile313, Phe317, Met318, Met351, Glu355, Glu359, Ile360, His361, Leu370, Asp381, Phe382, His396, Ser417, Glu459 and Phe486 is replaced by another amino acid, said mutated functional kinase domain being resistant to inhibition of its tyrosine kinase activity by N-[4-methyl-3-(4-pyridin-3-yl-pyrimidin-2-ylamino)-phenyl]-4-(4-methyl-piperazin-1-ylmethyl)-benzamide or a salt thereof, to the use of such polypeptides to screen for compounds which inhibit the tyrosine kinase activity of such polypeptides, to nucleic acid molecules encoding such polypeptides, to recombinant vectors and host cells comprising such nucleic acid molecules and to the use of such nucleic acid molecules in the production of such polypeptides for use in screening for compounds which inhibit the tyrosine kinase activity of such polypeptides.