Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): A calcium-dependent bacterial endonuclease (red) can degrade retroviral RNA (yellow ribbon) when targeted to virions via fusion to the retroviral nucleocapsid protein. Moloney murine leukemia virus gives rise to leukemia in mice, as evidenced by the infiltrated leukocytes in liver tissue of infected mice bearing a Moloney murine leukemia virus transgene (inset). A transgene expressing the endonuclease-nucleocapsid fusion protein provides a significant therapeutic effect to infected mice, reducing infectious virus titers, conferring increased longevity, and decreasing the extent of splenomegaly. Cover art by Bang Thoai Wong. (See related article on p. 7030).