Apoptosis in tumor xenografts as measured by TUNEL staining. Tumors harvested from mice after 21 days of treatment were fixed in formaldehyde, deparaffinized, and subjected to immunohistochemical analysis. Shown are TUNEL stains of CAG myeloma xenografts with short telomeres after intratumoral treatment with (A) PBS, (B) 227 (10 nmol), and (C) GRN163 (10 nmol) 5 times a week for 3 weeks. These are compared with TUNEL stains of HT tumor xenografts with long telomeres after intratumoral treatment with (D) PBS, (E) 227, or (F) GRN163 on the same schedule. Apoptotic cells are indicated in brown (arrows). Original magnification, × 40.