The cover shows many facets of genetics. Top row, Left: The DNA double-helix, here imaged in a scanning tunneling micrograph, is central to all genetics research. Right: Experimentation has shown that some social behaviors, such as nest cleaning by honeybees, is under genetic control. Second row, Left: Inherited disorders such as albinism, manifested here in a bullfrog, have provided many insights about the genetic control of metabolism. Right: Gregor Mendel's 19th-century work with pea plants elucidated the basic principles of inheritance. Third row, Left: Efforts to combat HIV, the virus that causes AIDS, depend on knowing how the virus expresses its genes inside the cells of the immune system. Right: The fruit fly is ideally suited for studies on the genetic control of embryonic development and organ formation. Fourth row, Left: The identification of mutations that cause unregulated cell division facilitates the diagnosis, treatment, and prevention of breast cancer. Right: HeLa cells, derived in 1951 from Henrietta Lacks, a woman who died of cervical cancer, thrive in the laboratory and are used in research worldwide. Bottom row: The replication of chromosomes (left) is a prerequsite for cell division (right).