Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Colony morphologies of Srf^/+ (top) or Srf^/ (bottom) embryonic stem (ES) cells were determined by scanning electron microscopy upon cultivation for 48 h in complete growth medium on gelatine-coated dishes. Note that individual cells within an Srf^/ ES colony established fewer contacts with neighboring cells than those within an Srf^/+ ES colony. (See related article on page 2933.)