Contents

17 December 1999
Vol 286, Issue 5448

About The Cover

Cover image expansion

COVER Artist's conception shows stem cells, the Breakthrough of the Year in 1999, under the microscope. The fate of many of the cells remains a mystery to researchers, as indicated by opaque cells. But under scrutiny, some stem cells reveal their ability to become different kinds of cells, including neurons, fat cells, muscle cells, and red blood cells. See the Breakthrough of the Year special section, beginning on p. 2238, and the Editorial on p. 2267 [Illustration: Steve Keller]