THE development of a multicellular organism involves a delicate balance among the processes of proliferation, differentiation and death. Naturally occurring cell death aids tissue remodelling, eliminates supernumerary cell populations and provides structural elements such as hair and skin. In the nervous system, selective cell death contributes to the formation and organization of the spinal cord and sympathetic ganglia¹, retina² and corpus callosum³. But cell death also occurs in several neuropathological conditions, such as amyelotrophic lateral sclerosis⁴ and Alzheimer's disease⁵. Therefore an elucidation of the mechanisms responsible for cell death is critical for an appreciation of both normal development and neuropathological disorders. Using a fos-lacZ transgenic mouse⁶, we provide evidence showing that the continuous expression of Fos, beginning hours or days before the morphological demise of the cell, appears to be a hallmark of terminal differentiation and a harbinger of death.