Cover photograph (Copyright © 2003, American Society for Microbiology. All Rights Reserved.): Increased numbers of apoptotic germ cells are present in the seminiferous tubules of sir2 null mice. Testis cross sections from wild-type (top left) and sir2 null (bottom left) mice were analyzed by the TUNEL assay to determine the frequency of apoptotic cells. Following exposure to 3 Gy of X-irradiation, the number of TUNEL-positive cells was increased (right panels) in both wild-type and sir2 null mice. Significantly more apoptotic cells were present in the sir2 null testes than in the wild-type testes (P < 0.05 by Student's unpaired t test). (See related article on p. 38.)