Cover photograph (Copyright © 2008, American Society for Microbiology. All Rights Reserved.): Collapse of DNA replication forks can produce cruciform DNA structures, termed chicken feet, as represented in the foreground. We propose that the homologous recombination protein encoded by Xrcc2 is important to repair such collapsed replication forks in developing lymphocytes. In the absence of Xrcc2 function, chicken feet are sensed as unrepaired DNA double strand breaks (as revealed by staining for phosphorylated histone H2AX [upper left]), leading to S-phase arrest and eventually chromosomal damage (shown by karyotype analyses [lower right]). (See related article on page 2295.)