p53-dependent gene expression. (A) p53-dependent gene expression response of CLL cells in vitro following treatment with fludarabine or irradiation. Gene expression changes at 24 hours (1 day) and 48 hours (2 days) are displayed for each gene relative to its expression in control cultures. In CLL patients 15 and 53, fludarabine treatment and irradiation were cytotoxic, whereas no effect of either treatment was observed in CLL patient 31, who was clinically resistant to fludarabine. (B) Immunohistochemical staining for p53 in CLL 31, demonstrating overexpression in approximately 30% of tumor cells. The image was obtained using a Zeiss Axioplan 2 microscope, 100 ×/1.4 N.A. planar apochromatic oil immersion objective, and MicroMAX digital camera (Princeton Instruments, Trenton, NJ), and then processed with IPLab Spectrum (Signal Analytics, Vienna, VA) and Adobe Photoshop Software.