Cover legend: Pak1 activation mirrors axon specification. Rat embryo hippocampal neurons maintained in vitro for 2 d exhibiting restricted activation of Pak1 (green) only in the longest neurite, which is considered the most likely future axon. Pak1 activation was detected using a S199/204 phosphospecific antibody. Neuronal identity was confirmed with immunoreactivity for βIII-tubulin (red), and the nuclei were visualized with DAPI staining (blue). This is the first time Pak1 has been shown to play a role in the establishment of neuronal polarity. For more information, see the article by Jacobs et al. in the August 8, 2007 issue (pages 8604–8615) (http://www.jneurosci.org/cgi/content/abstract/27/32/8604).