Hsp90 expressed in ZAP-70⁺ CLL has increased binding affinity for 17-AAG. (A) Protein lysates from different samples were evaluated for their binding affinity to Hsp90 inhibitors in a competitive binding assay using a biotinylated geldanamycin (biotin-GM) probe and increasing concentrations of 17-AAG. (B) Hsp90 derived from ZAP-70⁺ CLL cells showed a higher binding affinity for 17-AAG with an IC₅₀ of 31 nM (SEM ± 2), whereas Hsp90 from ZAP-70^-CLL cells, and normal B and T cells had an IC₅₀ of 300 nM. Purified recombinant Hsp90 had an IC₅₀ of 600 nM. This experiment was reproduced 3 times.