Contents

10 November 1989
Vol 246, Issue 4931

About The Cover

Chimeric mice derived from embryonic stem cells and progeny carrying a targeted gene disruption at the c-abl locus. The agouti coat color is contributed by the embryonic stem cells. Three male chimeras of different host genetic backgrounds, CD-1, C57BL/6, and MF1, are shown clockwise from the top. C57BL/6 chimeras have transmitted the c-abl mutation to their progeny (for example, center left). See page 799. Meanwhile, difficulties in working with human stem cells have spurred research on an alternative approach—human endothelial cells as a delivery system for gene therapy in medicine. See page 746. [Photograph by Vergil Sweeden]