<qa>  What is the effect of ZD1839 on tumor growth delay?In preclinical studies, ZD1839 causes tumor growth delay in a range of cancer cell lines and xenografts, and tumor regression has been observed at higher doses. ZD1839 was also shown to inhibit proliferation in DCIS together with significantly increased apoptosis in these tissues. Data from these different cell lines and xenografts suggest that ZD1839 may act via dose-dependent growth inhibition and by increasing apoptosis, and may even potentiate the activity of cytotoxic chemotherapy. 