The concentration of drug required to reduce cell viability by 50% (IC50) for gefitinib in ZAP-70+ and ZAP-70− CLL cells was assessed using the MTT (3,3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) cell viability assay. The gene discussed is ZAP70; the disease is B-cell chronic lymphocytic leukemia.