A three zinc-finger protein that binds specifically to the cDNA representing the unique fusion gene BCR:Abl, associated with acute lymphoblastic leukaemia, has previously been characterised. At this breakpoint, a sequence homology of 8/9 bp exists between the BCR:Abl (fusion) and c-ABL: (parental) target sequences. We show that the three zinc-finger protein discriminates poorly between the fusion (BCR:Abl) and parental (ABL:) sequence (K:(d)s of 42.8 and 65.1 nM, respectively). In order to improve the discriminatory properties of this protein, and to demonstrate the utility of current zinc-finger databases, we have added a fourth zinc-finger to the original three zinc-finger protein. This fourth finger recognises a 3 bp subsite derived from the BCR: portion of the breakpoint and is not present in c-ABL: This novel four finger protein, which now recognises a 12 bp sequence, demonstrates improved specific binding to BcrAbl (K:(d )= 17 nM). More significantly we have shown that there is now enhanced discrimination between BcrAbl and ABL: sequences by the four finger protein than the original three finger protein.